The gameboy micro is a thing of beauty.
I recently acquired one and I figured I could share my take on it, as a Zerey's modded AGB owner.
As I said, it's a thing of beauty, a "unibody" metal handeld (yep it's anodized colored metal, for those not familiar with it, except the removable plastic faceplate), so compact and weightless in your pocket. If you're into GBAs, you should own at least one, no matter what.

BUT

I feel you should only consider it as a SECONDARY gameboy advance. Why? Because L and R are painful to use. Don't even think to fire Missiles (R+B) in Metroid Fusion without hurting yourself or missing your targets more often than not. Start and Select are also more difficult to press than they should be. I see lots of people in "what GBA should I buy"-threads going "not the Micro because the screen is too small", but as I see it, the buttons are by far the worst issue. Screen looks ok to me.
It just can't be an "all around" GBA-era gaming machine. Games that heavily rely on L, R, Start and Select are at least uncomfortable to play on it. So, as I said, definitely buy one but don't expect it to be your ultimate GBA machine. It's only OK for those games that doesn't require you to quickly press L/R (like jump-spinning in Super Mario World) or pressing L/R plus another button (like Missiles in Metroid) or pressing L/R/Start/Select all the time (like pressing Select to drop the current power in Kirby). You should have no problems with slow paced or turn based games.

Now to the actual comparison (GBM = Gameboy Micro, zAGB = Zerey Zhang's modded AGB-001)

1) Screen
The GBM's screen is smaller but still usable, sharp, good colors, I expected it to be somewhat FASTER (in terms of pixel refresh) than the "generic chinese LCD" of the zAGB (I don't expect AGB modders like Zerey or Rose Colored Gaming to destroy an actual AGS-101 for every AGB they mod, your mileage may vary if you own a modded AGB with an ACTUAL ags-101 screen you modded yourself), but I don't really notice it. Truth to be told, the slowness of the zAGB pixel refresh is only noticeable to me when loading GBC games, but for obvious reasons (the GBM can't play GBC games) I couldn't test them. If you're concerned about pixel refresh speed or whatever you call it (basically how fast the screen refresh in order to depict movement without dark trails), your best bet is probably a FRONT (not back) lit AGB-001 or an AGS-001. Old school non-backlit screens are said to be the fastest.

2) Brightness
The GBM has 5 brightness levels (selectable with L+volume buttons), the zAGB only 1 (I read other modders include a 2-level brightness switch, I personally don't feel it's necessary). The zAGB brightness roughly equates to the 3rd (middle) level of the GBM, and it's OK. The 2 upper levels of the GBM can feel too bright at times (indoor) or to the very least are a waste of battery life, level-3 is OK. 4 tops. 5 is useless.

3) Audio
The mono speaker on the zAGB is loud enough. The headphone output is dirty and low, feels like the 90s.
The mono speaker on the GBM is NOT loud enough, I have a fan in the room right now and I struggle to hear it even at max volume. In complete silence is loud enough. The headphone output is clean and loud, feels like the 2000s.

4) Battery Life
I don't know how much my used GBM has been abused in the years, and I don't feel like spending for a new battery already, so what I say refers to an original battery that has undergone a random and unknown amount of use/abuse in the years. It struggles to lasts an afternoon (lunch to dinner) of intense play, at middle brightness. On the other hand the zAGB with a pair of Sony Cycle Energy AAs can easily last an afternoon of intense playing/testing and still have some juice. Plus, you can just swap the AAs and continue playing. And you will still be able to feed it fresh AAs 20 years from now. Will aftermarket GBM's batteries still be available in 20 years? I don't know. Probably not. Sidenote: if your used GBM doesn't come with a charger, you can just buy a USB to GBM cable and charge it with any USB charger.
The GBM has 2 LEDs under Start/Select that flash blue during boot if the battery is OK, flash red every now and then during use when the battery is like under 20%, and stay solid red when the battery is approaching 0%. When you're charging it the LEDs are solid blue, and turn off when recharge is completed.
The zAGB has a power on LED that is solid green when the battery is OK and turns solid red when the AAs are running out of juice.

5) Grip and Buttons
As you may guess from the preface, the GBM in not the most comfortable handheld ever. As far as grip is concerned, the zAGB allows for a natural and comfortable rest of the index fingers on the L/R buttons. The GBM is the exact opposite: if you want to be always ready to press L/R in a split second, you'll have to hold it in a weird, painful and tiring way, at least for regular sized adult hands. Buttons are just too close. And you can't even press L/R on the outer corners: they actually depress on the INNER part. Yep. Imagine that and what weird grip you need to come up with to be able to press any button in a split second. Metroid, Super Mario World, Mario Kart (drifting and using power ups), etc. are all uncomfortable. Those are big ones. This can't be your ultimate and only GBA. Start and Select are also bad offenders, while not being as bad as L/R. You have to HUNT for them every time. It's a game within the game: trying to hit those super thing start/select buttons. Annoying.
A/B buttons are slightly springier and better on the GBM. (keep in mind that these are both used machines who underwent who knows what in the years)
The d-pad, maybe slightly better on the GBM. Better up/down and left/right "separation" on the zAGB (like for those games in Warioware Inc. where you have to press up/down alternatively, it's easier on the slightly bigger zAGB's d-pad).
If I had to choose one (as far as FACE buttons are concerned, not the stupid L/R), I'd go for the GBM's d-pad and A/B. They feel more like the 2000s. The AGB face buttons feel more like the 90s.
Speaking of grip, weight should also be mentioned. The GBM is EXTREMELY lighter than the AGB. The AGB is tiring to hold, you're better off resting it on your belly while sitting laid back. In bed, it's tiring to hold the AGB mid-air. It's never tiring to hold the GBM. That's a big plus for the GBM.

6) Portability
You don't wanna leave the house with an AGB.
You do wanna slide your GBM in the front pocket of the skinniest jeans you got.
Enough said. Size wise, the GBM is probably the most pocketable handheld ever.
On the other hand, a zAGB and a bunch of AAs could be a good idea for a long trip. The GBM will die on you in 4-5 hours. Since USB to GBM cables are available, you could charge it on the go with those external battery thingies for phones/tablets, though.
Under this portability section, I should also mention that holding an AGB in public could be a bit weird, while the GBM design is a bit more "timeless" and could be carried today or 10 years from now without looking particularly weird. The AGB is a bit on the toy side, the GBM is a polished slab of metal.

7) GB/GBC support
The GBM can't play GB/GBC games, but I don't like how GB/GBC games looks on GBAs anyway, so it's a non-issue for me.
GBC games show dark trails of slow pixel refresh on the zAGB screen.
Just play GB/GBC games on a modded frontlit GBC, that's the best way.

8) External peripherals support (like tilt sensors)
I don't own any of those, I can only note that the cartridge slot on the AGB is centered, while it's moved to the right side on the GBM (and it's upside-down like on the SP), don't know how this translate to usage with tilt sensors (like for Warioware Twisted) and other peripherals.

9) Available colors
The AGB is available in many solid or transparent colors, and now infinite new combinations thanks to modders like Rose Colored Gaming.
The GBM is commonly available in the following colors (for a complete list of every GBM ever existed look here, I only list the ones I commonly see on sites), I took the liberty to list them from my favorite to least favorite and briefly comment on each one:
Blue: the one I own (and only one I saw in person), nice blu, matches with the blue LEDs under start/select, jack of all trades color while not being the banal silver
Pink: nice shocking pink, black buttons are the perfect complement
Purple: rare, very nice
Red: apparently very rare, a must have in a "GB to 3DS all red collection"
All Black: rare
Famicom: I don't buy into the hype or nostalgia so I won't put this at the top of the list, sorry. It's a nice collectible, it's history, it's shiny. It's also very expensive and less functional than your average 50$ used GBM. Why? Because you'll be always nervous about damaging it. I can see how it can be regarded as "beautiful", but its aesthetics don't feel like something you would wanna use on a daily basis. It doesn't blend with the surroundings (unless you live in some sort of gold+dark_red chinese emperor palace or you have a room full of Famicoms). It doesn't blend with what you play onscreen (except Famicom Mini series). It feels like it belongs to a museum, it'd feel weird to pick it up to play some Warioware Inc. on the toilet.
Green: meh
Silver+Black: Silver body, black faceplate, I think it's pretty stupid.
Silver: Boooooring. Banal. Cheap.
Mother3: The last spot is taken by the priciest GBM of the bunch. Owning this can cost you as much as collecting every other color. Yep. And to add insult to the injury, you'll have to endure that STUPID STUPID STUPID fake discoloring. It's so stupid it deserves the only picture in this post.



Again, fantastic collectible. But stupid on a daily basis.

10) What about other GBAs?
GBA SPs frontlit (AGS-001) and backlit (AGS-101) may be practical to carry thanks to the clamshell design that protects the screen, but I don't like the fact that you need an adapter to use 3.5" headphones, or a particular splitter if you dare to charge them and use headphones at the same time (what was Nintendo thinking?). And, like the micro, they won't work 20 years from now if fresh internal batteries won't be around anymore. Whereas there will always be fresh AAs for the zAGB.

Cheers

 

The original Game Boy Advance is still by far the best form-factor there is out of all the Game Boy Advance revisions, at least in my honest opinion. It's a shame that Nintendo never released a backlit version of that system - playing on it would be far more comfortable if you didn't have to sit on the face of the sun to see the screen.

 

Meh, I want to use my gameboy micro more on the go since that thing is a fucking stealth console, but I stopped using it because I'm worried the faceplate on it is going to get scratched. Its my only faceplate =/

 

i've always wanted to try a gameboy micro, however, you only find them used anymore... and people keep trying to sell their scratched up micros for upwards of 50€, it just doesnt seem worth it.

 

Kinda my point
Both for the famicom and the mother3.