The only reason I wouldn't give it a 9.0 is the lack of online capabilities.
What "Guitar Hero" has to offer was much more than "Rocks the 80's", it has the usual career mode in four difficulties, unlockable characters, lots of bonus songs to buy and some other stuff as well while the second game even added more on that. The first Guitar Hero has lots of great songs with only one drawback that stopped me from playing it more than the others, it was the "hammer on" and "pull up" techniques. Those are techniques used to make playing the harder difficulties easier and enjoyable, unfortunately in the first game they weren't executed very well but the developer did a great job tweaking them for the other installments.
What is really fun about Guitar Hero game is that the more you play it the more you enjoy it, unlike other games that you mostly play once or twice and not more than that. In Guitar Hero the real fun starts when you get used to the hard difficulty where you start to feel like playing a real guitar and then you go over the songs that you couldn't finish before and play them over and over till you can nail them down at the expert difficulty which is the ultimate fun in guitar hero. If you combine my playing time in all of them it might reach 60 hours or more and honestly I'm still playing them from time to time and still enjoying them.
As for "Guitar Hero III" everything almost doubled from difficulty, number of songs in career mode, unlockables, and the bonus songs to buy along with the most difficult song in all guitar heroes "Through the Fire and Flames" which is luckily not part of the career mode but a blast to play. An extra mode was added also which is the boss fight where you have to play against the CPU and blow him with power ups you gain if you play correctly. Not only that, if you want to boost the value of your game then I recommend you buy either the PS3 or Xbox360 version as it contains online multiplayer and extra songs some for free and others to buy for a reasonable price.
Right now I can simply say that Guitar Hero is the best rhythm game I ever played to date, and I'm impatiently waiting for the two upcoming installments "Guitar Hero: Aerosmith" and "Guitar Hero IV" which I will definitely buy on a next generation console to enjoy their online capabilities. I really can't roll out "Rock Band" which is supposedly the best party game so I will post my opinion as soon as I lay my hands on it.