Its been a long, long time since I've had a game that I didn't know I wanted immediately or would otherwise avoid.
There is not that much creativity in games these days(outside of arcade) so either a game is going to be polished or a worthless clone and you can usually tell.
I use reviews to help me figure it out if I'm on the fence, or sometimes if you know the guy behind the counter is smart you can ask him but a lot of times its not a good bet.
Though if its in the bargain bin I usually don't care, I've gotten a few surprising gems out of there but also a bad apple or two but I kinda chalk these up to "hey it only cost me 5 bucks" deal.
I've only been disappointed in paying for one full priced game in the last couple years that I remember. Though if you knew which one alot of people would look at me funny.
Playing as many demos as I could get my hands on helps. In most cases it gives you a feel for how the game will be.
