99% of the time, I'm happy with my purchases. Usually, before I buy a game, I watch a few gameplay videos, check out the TA thread for it, and more, and that's usually when I decide whether to buy it or not.
The ones I actually pay for are usually good enough to justify the 69p I usually pay. The worst that usually happens is that I pay for a game that isn't bad, but I just don't play. Offhand, that's Reckless Racing, Slingshot Racing and Chopper 2. Paper Titans and The Curse were a bit disappointing too.