I can see only three (possible) negatives to using the 600's for gaming:
1. Potential mic issues. Being open means that some of the audio from your headphones can leak into your mic when trying to communicate. I wouldn't consider this a big deal, though. It only matters if you really, really, really want a clear channel for your voice. It will be significantly better than using a mic with speakers, but I've done that in the past without too many complaints.
2. Less bass. This is more of a gimmick, but I can enjoy a lot of strong bass to "feel" what's going on around me...actually, this is completely a gimmick. I would only consider getting more bass out of my gaming 'phones if I also needed...
3. Isolation. This is also a limited scope item. Again, the 600's are open, so you are still going to hear things around you. Normally you would only care about isolation if you played in a noisy environment, but isolation could also be good if you were really into a survival horror game, since it could keep you closer to the game world.
So yeah, these things are really minor and subjective. The 600's should and will be great. For any positional audio stuff, you either need a third party program to mix your sound down to two channels, or you need your game to do it (many PC games have audio settings for headphones).
I just got my 600's recently, so I haven't gone through the full round of games yet, but I love the sound quality, and I would have no issues using them for everything on my desktop.
EDIT: for reference, my other pair of headphones--which I have been using for gaming--are ATH-M50's. They are better at isolation at the expense of some sound quality.