I think everyone is hooked on this game because well... it's a great game.

I mean, basically you get to "play" the songs of your favorite artists and at times, you do feel like a Rock God (tm). It's just a lot of fun.
Initially I was the same way. My buddy had GH1 for the PS2 and kept trying to get me to play. I kept seeing how stupid he looked playing it and thought "no way." Finally, I sat down, grabbed the guitar, and fired up Joan Jett's "I Love Rock and Roll," and have been hooked ever since.
The 360 offers the best of the series since it has downloadable content, a really solid guitar, the leaderboard (which GIJoe is rubbing my face in currently

) and the ability to play with friends if you have the controllers.
Mostly, I just find it a lot of fun. I've had "Woman" stuck in my head for days now because of it.