Even if you don't own a Playstation 2, it's worth purchasing the console and the game. It's that good.
The first Guitar Hero, which was released in November of 2005, sold over 1 million copies. For a game costing $60.00 and released only for a console (the PS2) which was five years old, that's impressive.
When you play it, you will understand.
If you want to read more, check out the reviews on Metacritic.