Guitar Hero by Harmonix is the best music oriented PS2 game since Amplitude. I was into Frequency and Amplitude since the minute they were released. Hours upon hours of my life have been spent trying to top scores and finish the most difficult songs on expert difficulties. Now take the fun and formula from both these games, add an inventive guitar controller, and you've definitely got a winner on your hands.
The music featured in Guitar Hero is spot on. You've got everything from Black Sabbath, to Boston, to Sum 41, to Pantera. What's even more interesting about it is the fact that every song is a cover, but 90% of the time you can't even tell. The only songs that don't sound quite right are Megadeth's Symphony of Destruction and Incubus' Stellar.
I cannot think of any negative aspects of the game. The guitar controller only takes about a half-hour to get the hang of, then after that it's smooth guitar-shreddin' sailin' from then on.
If you're at all a fan of rhythm games (or fun games in general), shelling out $70 on Guitar Hero is a no brainer.