The people there are pretty cool, and you can get some decent deals there. It's nice if you're trying to trade for a little above market value, but like Moses said, you'll get low-balled a lot, and people will ask for ridiculous prices on games that are worth less than $1 on ebay.
Overall though, gametz is a lot like a dating site. You can offer someone a fantastic deal and they STILL won't trade. It's hit or miss, but there's nothing wrong with trying it out. I only have 14 trades there, and all of them but 1 went smoothly. So it's still a pretty good site.