From a guy who's only played the guitar part of the game (but wants to buy the drums):
What I'm planning on doing is buying the PS3 version whenever I get enough money to buy a PS3. But anyways here is a review for the guitar section of the PS2 version:
Most of the songs are rather boring until you get to the like the last 15 (out of like 58) but if you really like those songs it is probably worth it. The system is a little changed from Guitar Hero: Harmonix has made the notes a little easier to hit than on GH2 (but still not as easy to hit as GH3); the notes are rectangular instead of round (an easy thing to get used to except instead of hammer-ons having white in the middle they are just smaller); you can gain star power while using it (an improvement for sure); some songs have a freestyle jamming session at the end which is pretty damn cool.
I would give just the guitar part like a 7, but I can't imagine how much fun it would be to try the drums or play with four people (I just can't afford that). Plus if you have the Fender Strat controller you can use the solo fret buttons and not have to strum. Also if you own the 360 or PS3 version there is tons of awesome songs (more than in the actual game) to download from the online stores.