Worst ever: Guild Wars Nightfall Collector's Edition. Played it for maybe 5 hours. Couldn't get into it at all. Already had Factions, which was.. allright, but I HATED the city environment and stuff, so I figured what the hell, my closest friends play it, I'll give it a shot. Didn't work.
Not great, but was on sale: Mass Effect 1 and 2 this past christmas Steam sales. Sure, they were cheap, and my brother love them, and I love DA:O, so what the hey, but no, I keep trying, gotten 9 hours into ME1, but it just doesn't grow on me. I like the story, some of the characters and the dialogue, but the gameplay.. and OH GOD THE COMBAT! It's just so bad.
Best impulse buy ever: Humble Bundle 5. Didn't much like Sword & Sworcery, or Lone Survivor, but Braid, Super Meat Boy, Psychonauts and the glorious awesome that is Bastion. Pure love.
Honorable mention to Just Cause 2, which I got for 5$ on Steam. Bad story, but the pure, unadulterated FUN of it is great.