GH3 makes you want to get better and better until you really are a Legend of Rock.
The song selection has a very wide range and most of the songs keep you interested enough to want to play them over and over. However, they can't all be good. Once you get to Hard mode there are more and more songs that will require some practice. The problem is that when it's a song you really don't like playing or even listening to (i.e. Raining Blood) the game can be a little frustrating.
Practice makes perfect in GH3 and it's amazing how quickly you can go from being just able to pass something on medium to completely destroying the songs with near perfect scores. It's also difficult enough that in order to beat the game on Expert you've really got to be great. I've had it for about 3 weeks now and can beat some of the songs on expert, but am really focusing on upping my ratings on hard.
GH3 is highly addictive and a lot of fun to play alone or with other people around. I've never played a guitar battle against another person, but I think it would be fun (as long as you were appropriately matched). I think that for 100 bucks it's a great buy and worth every penny. I would highly recommend it to anyone young or old.