Rock Band 2 (Xbox 360, 2008)

Rock Band 2 is worth the $50-$60 you'd have to pay retail for.

First and foremost, you no longer just play songs... RB2 gives you challenges in the form of tours. These tours can either be unlocked by yourself (or with band mates -- additionally, not all band members have to be present for you to progress through the game). Having a string of songs makes the game more challenging and interesting than just continuously picking.

Additionally, you can specialize your character if you choose to make one. This might not be a big thing to some, but it is pretty neat to see a virtual version of you solo'ing or drumming away. Also, you have to unlock certain areas by playing, gaining fans, or gaining star ratings in order to get special gear. This encourages even more game play, as you have to play at specific cities in order to win certain merchandise.

I don't know if you can really say there's "replayability" because the game never really ends. I spent 8-10 hours straight playing with my college buddies, and we just couldn't get enough. Even the annoying songs are entertaining because of how much they challenge you on each instrument (even bass).

I highly recommend you get a second guitar though in addition to the mic, drums, and guitar. In doing so, see if you can get an RB2 wireless controller with auto-calibration (this takes care of visual and audio discrepancies). The $70 spent helps you get to game play faster (especially if you have to deal with LCD/Plasma screens).

Hope this helps; feel free to message me with questions!