Guitar Hero III has a few flaws, but it is still the game you remember if you've played the others in the series.
Neversoft managed to keep in place the old Guitar Hero feel of the game, just as you remember it if you have played past games. In fact, minor tweaks to the game engine were added. The game definitely looks better graphics-wise, and the rock meter/score meter/fretboard have gotten a facelift. Some veterans of past games may encounter a sort of "culture shock" because of the different way the game controls.
However, practice mode is still intact if you need to overcome issues you may have playing songs, or if you just want to see how it plays out. The career mode is very nice, but it feels almost like nothing different was really done with it, aside from adding a few little mini-story cutscenes to your band as they progress from venue to venue and incorporating the battle system into boss battles. While this is not necessarily a bad thing, it doesn't feel like it adds much in the way of content. Co-op career is something new and nice, and is great if you have an extra guitar controller and person to play with you. Extra encore songs that are not unlockable on the single player career can be unlocked here. Quickplay mode is the same as it always was, nothing new here, with the exception that the venues, guitar, guitar finish, and the character chosen when a song is started really ARE random now as opposed to blatantly pre-programmed as they were in previous games. In training mode, nothing is different either aside from an extra tutorial on battle mode.
Multiplayer is where the game gets most of it's replay ability, unfortunately the only new thing here is battle mode. Sadly, in the PS2 version of Guitar Hero III, there is absolutely NO online multiplayer to be seen or heard of. This drastically kills the replay value of the game, unless you always have someone beside you to play with. This is not to say that if you're the completist type you won't have good replay value, which is where difficulty comes in.
The game feels a tad bit more difficult than previous Guitar Hero games. This is both a good thing and a bad thing. The Expert difficulty fanatics will have a challenge they never dreamed of, but on the flipside, those who play on easy or medium difficulties find that it's noticeably more difficult, especially towards the ending songs in the career, and a few of the bonus songs get difficult as well. If you are new to the Guitar Hero series, it is not suggested that you start with Guitar Hero III unless you have someone to really tutor you and show you a few advanced moves early on, as this game makes consistent use of them. You're liable to get hand cramps really fast otherwise.
The game, if you buy the bundle, comes with a wireless controller modeled after a Kramer Striker. If you have your SG controller from previous Guitar Hero games, you're better off not buying the bundle unless you REALLY want a wireless controller, or need a second controller for competitive/co-op play. If you do not have it then it's the better way to play the game, although the game can still be played using a Dual Shock analog controller. Some have reported issues with the strum bar being oversensitive or fret buttons not registering the way they should. Personally I have not experienced this problem. The controller feels bigger and heavier than an old SG controller, and I prefer the SG controller to it unfortunately.
The game uses a bit of subtle advertisements, usually in the venues. But when you have a guitar plastered with the Axe body spray logo on it, that will most likely turn you off. What possessed them to do this is beyond me, since the game has no online multiplayer, they don't need ads to pay for the bandwidth servers use.
There are a few things that bring the Guitar Hero experience down, including the fact that the game almost hasn't changed much, but for the most part, it's the game you remember and love to play. Pick this one up if you get the chance and love the series, but if you're new to the series and are just beginning to play, go for one of the older Guitar Hero games to learn how it's played first. If in serious doubt, it's worth at least a rental.