I've used all three to some extent. Admittedly, I only had the EZ-5 for a week (before passing it on to a friend whose child couldn't cope with the concept of NOT pressing the X button or mashing the keys on an R4), but I have a few observations to make.
With the EZ-5, after you update the loader it's probably a damn good idea to remove the update file from the Micro SD - in case button-mashing kids start a loader update and turn the device off part-way through. Don't know for sure, I imagine that a partially-installed loader wouldn't be particularly useful...and I don't want to waste $30 finding out, thank you very much. That's not an issue with either the R4/M3Simply or M3 real.
EZ-V has pretty crappy download-play support. Both the others beat it hands down in that regard. But is *does* work seamlessly with the 3-in-1. Apparantly the 3-in-1 isn't too tough to get working with the others either, but it just works out-of-the-box with the EZ-V. Could be important for those *really* interested in GBA titles.
Both the M3 Real and the EZ-5 have the ability to take you directly to a menu from which games can be launched - the R4/M3Simply needs you to press a button or touch the screen to select what you want to do (run DS software, launch Moonshell, load something from Slot 2).
If the M3 Real has a counterpart to the "X-Button of Doom" of the R4/M3Simply, I haven't found it yet. There's no equivalent function that I could get to in casual button-pressing on the EZ-V either. In the "Kid-Proofable" category, both the EZ-V and the M3 Real beat the R4.
My 7-year-old has had an R4 since late June. He's also a cheat-junkie, pretty sharp, and pretty flexible when it comes to dealing with different interfaces. Before I let the EZ-V go to its new home, I let him have a play with it and he thought it was in some ways a lot easier to use than the R4. He's also had a poke at my M3 Real, and his verdict is that it's a little harder to use than the R4. Sure, you can set it to bring up the game list first up (in fact, that's the only way he's seen it), but he found the UI a little more cumbersome. I do too. Nothing major, but things like the drop-down menu for the soft-reset on a per-game basis (rather than the right shoulder button as a persistent setting) are a little jarring. Okay, I appreciate that some games use the L-R-A-B-X-Y mash for other things, and some people are after flexibility, but persistent settings for the mash-type and a shoulder-button or touch option to turn it on or off persistently might sit better with us. In terms of overall ease-of-use, from easiest to hardest, it's EZ-5 then R4 then Real in our opinion.
Of course, EZ-5 then R4 then Real is also the order of least to most flexible and feature-filled too, for most things - the question is, is all that flexibility really useful for most people? Probably not, at least not with all the half-baked extras like the organiser and the media player (note: again, this is purely subjective, and I seldom use Moonshell on the R4 either). One area where the EZ-5 stands ahead, though, is the ability to load it from the DS's own menu - there may be some PictoChat addicts out there for whom this is important, and this also gives you the ability to be a download-play recipient without having to remove the cartridge from the DS... given the issues some people seem to have with flashcard contacts and wear and tear on the DS, this could be a big plus for some users.
(on an off-topic tangent, something that would REALLY improve the concept of an organiser on the DS would be a revised DS console that had Bluetooth - and decent synchronisation tools to go with that. Sure, you could probably cobble something together to FTP files from your desktop to your DS via wi-fi, but there's not the kind of ease-of-synchronisation that would REALLY make people give up their iPhone or ipaq or smartphone or whatever... sure, it's cool, but a lot of what people do with these things these days makes futzing about with TF cards a pain for day-to-day synching. If someone REALLY wanted to use an organiser on the DS for REAL work, I'd probably have to suggest that they either use a G6 Real or an F-Card - at least that way, you can stick the whole cartridge in the reader.)
Can you do soft-reset on the EZ-V now without patching games? I think you might be able to, but it has to go through that whole moonshell loader startup again.
So... what wins?
If I wanted SDHC support, the Real is the only option out of those three.
If I wanted something that allowed me to be a download-play recipient without having to remove my cartridge, the EZ-V is the only option out of the three.
If I want to host download-play games, I can't rely on the EZ-V for serving up all download-play titles.
If I don't like the "X-Button-of-Doom", I need to consider something other than an R4 - or resort to an older firmware that lacks the delete function, but also has less download-play compatibility and for which a lot of things would need patching. Might as well just get an EZ-V if that's the main deciding factor...
Oh, and the Real also doesn't by default display files in alphabetical order - it displays them in the order that they were written to the media, so it's by timestamp. That doesn't strike me as quite as nice as either the R4 or the EZ-V.
The Real has more "extras" in the included software and its options, but I'm not sure that for most people they're worthwhile. If you had an M3 slot 2 cartridge, the ability to launch Slot 2 in NDS or GBA mode could be important. Other than that, it's just an added layer of confusion for someone who might just want to play a GBA cartridge they've already got or have borrowed from a friend and then has to think about not just where the slot 2 loader is but also which one to use. Try explaining *that* to a 7-year-old some time, especially when you don't have any hardware that would take advantage of that distinction.
The Real and the EZ-V both seem to take longer to start up than the R4 too - and the EZ-V has all the diagnostic crap from Moonshell displaying while it loads.
So, what's *my* preference?
For most of what *I* want to do with it, the R4 is still my cartridge-of-choice. It does the things that the EZ-V doesn't that are important to me. None of the extra features, options or UI complexity of the Real are important enough for most of the people I know - and for all I know, by the time SDHC becomes important/affordable enough, the TT may well prove itself to be a worthy successor to the R4. I'll probably hang onto my Real for a while yet as it'll be interesting to see where the developers take it, and it's good to be able to look at the relative strengths and weaknesses of different cards close-up, but if asked I'll steer most people towards an R4 with an occasional suggestion that the EZ-V might be more appropriate for their particular application. And if you don't really know what you want, the R4 is probably a pretty safe all-round choice. But weigh up your options, and what's important to you - you're not me, and we don't know each other from a bar of soap, and I have no idea what's important to you or not. Good luck, and remember - all three of them are "good" cards, and all three of them have great features and all three of them have some pretty godawful ones too (IMO), it's just a matter of what you can tolerate and whether you've seen anything else to compare them with.