Any cons? None of the cards available are perfect yet, they all have cons of some type. The G6 Real does many things very well, and I think it is entirely worth the asking price! But because you asked about cons, I will only talk about the bad stuff below.
List of cons:
(1) It's biggest weakness is that there can be occasional file corruption on the card, forcing you to reformat and then copy over the latest image again. This is somewhat rare, and nobody seems to be 100% sure WHY it happens, but it's worth knowing that it CAN happen.
Personally, I think that file access when using certain homebrew programs is the culprit -- because that is the only time I've bumped into this problem. If so, then a software tweak to the DLDI code could fix the problem in a future firmware release.
(2) Homebrew support isn't perfect. It is very good, but there have been two or three old homebrew apps that didn't work for me (and some that didn't work because of problems with the homebrew code -- not the G6 Real card).
(3) Limited storage space. You won't be able to expand the onboard memory after buying the card, so be certain that you are happy with 1 or 2 GB before ordering.
(4) You are limited to holding 4 skins at a time on your card. Not sure why you'd need more than that, but some people apparently want a lot of skins to be available.
(5) Price. This card isn't cheap... is it worth it? I think so. You'll have to make your own choice.
(6) The built-in PDA functionality isn't very good right now. There is lots of room for improvement here... hopefully the dev team will work to make this (potentially awesome) feature more useful in the future. Until then, you are better off downloading the DSOrganize homebrew application.
That's really all that I can think of, and some of those "problems" are really stretching for something to complain about.
Edit: Oh, I'm still using firmware 2.3. Apparently there is new firmware available, but I haven't bothered to upgrade because 2.3 is working quite well and I haven't seen a list of what's actually changed yet.