Guitar Hero. While I was wasting 8 hours of my life waiting for a Wii on launch night, I finally got a chance to play Guitar Hero (a game I thought I wouldn't like because I can play real guitar (and drums) very well). It turns out I liked it enough to go out and buy the first two games a week later. I picked up Guitar Hero 3 as well and enjoyed it a good bit, although a bit less I think. This all built up to trying out Rock Band. I bought it, set it up and quickly began to loathe it. The new guitar felt broken, and Rock Band drums were seemingly situated in the very bottom of the uncanny valley. A few days later, I pulled it apart, took it back to Best Buy and told them it was broken (which, in a way, it was). Regrettably they had no units left to replace it so they had to give me all my money back. Suckers.
Since then the only temptation I've succumbed to is Guitar Hero: Metallica. I thought it would be interesting and it was a little. In particular playing Cliff Burton's bass parts was instructional since their mixed so quietly on the albums you usually can't hear them. That's the last Guitar Hero I buy however.