Guitar Hero is the best game I have ever played. I play it for hours on end (seriously, I'm not joking). It's that good.
It is easy to play, it has great music, there are levels (unlike the crappy SingStar), and it's got a guitar-shaped controller (the Gibson SG) with a whammy bar.
The best songs on the game are:
"Cowboys From Hell" - Pantera
"Bark at the Moon" - Ozzy Osbourne
"Texas Flood" - Stevie Ray Vaughan
"Ace of Spades" - Motorhead
"Crossroads" - Cream
"Fat Lip" - Sum41
"Cheat on the Church" - Graveyard BBQ
Also, I heard some awesome tracks I had never heard of before:
"No One Knows" - Queens of the Stone Age
"Ziggy Stardust" - David Bowie
"Unsung" - Helmet
"You Got Another Thing Comin'" - Judas Priest
Basically, it's got a wicked soundtrack.
Oh, and to top it all off, it's got a Gibson Explorer on the game.
Best guitar ever. Period.