Prototype 2. I almost cancelled the pre-order, and ended up wishing I had. The game wasn't horrible, but it wasn't great either. The graphics were ugly; too much brown. I didn't like the main character. The combat system was a mess. I mean, there were so many moves that you ended up hitting x+y, b+y, etc. for a lot of them, which made it very awkward. Plus, the moves were so similar it felt button-mashy. Mainly, the game was very repetitive and not much fun. If I had paid $10 for it instead of $60 it would have been fine.
That was pretty much the end of pre-orders for me. I think I've only pre-ordered Halo Reach, and Borderlands 2 since. I knew I was going to play both of those, and the pre-order bonuses were enticing.
Didn't pre-order, but paid full price for Brutal Legend. I loved the metal aesthetic. I never could get the hang of the combat system for the big battles. Never finished it.
I've bought a lot of used games that I probably shouldn't have; shouldn't have because my back log has become so big I will likely never play many of them. I tried playing Mercenaries 2 recently. It took about an hour before I decided there were probably better games I should be playing instead...