Since I don't really buy games on full retail prices unless their gifts for others.
Killzone 2: Bought it for the multiplayer, but the sluggish controls really turned me off. Tried selling it to my friends, but I still see myself stuck with the game.
Another purchase that I somewhat regretted doing was the Neverwinter Nights 1 Diamond Collection. Bought it for the 11 year living with us, since his one of his uncle could stop talking about the game. He really didn't know what the game was about, aside from "killing dragons", and BEGGED me to buy it for him. Long story short, he played about 1-2hrs of it, got overwhelmed with the complexity, and shelved it. I could've used the money on something else...