Sorry for yet ANOTHER review on Guitar Hero 3, but it's my favorite game and I wanted to write a review on something. That being said, I will try to keep it unbiased and honest and write it in terms of what I look for in a music game.
The first thing I look at in a game is how hard the achievements are. And in Guitar Hero 3, the achievements are notoriously difficult. As of this writing, over sixty thousand trueachievements users have earned achievements in this game and only 330 have completed it. The achievements take significant guitar skill, and even if you're okay in that respect, you still won't be able to attain the full thousand without talented local friends. Also, the Now That's Impressive achievement (get 750,000 points on a song) almost requires that you have Metallica's Death Magnetic DLC package. Another notable difficult achievement is Big Ol' Pile of Wins. It'll take around eight hours of grinding (double that if you do it for both players.)
Now I'll talk about gameplay. I've always been kind of competitive and a completionist, and at first, I wasn't really interested in playing this game too much as I knew it would require many many hours of practice before I was as good as I would want to be. I didn't plan on spending that amount time in this game. But eventually I found out that GH3 was addicting, time and time again I find myself thinking "just one more song... just one more song..." Eventually I found myself getting better, and the better you get at Guitar Hero, the more fun it becomes. There are some very difficult songs in this game, that's for sure. But no matter what skill you are, there are always some songs in some difficulties that will be right for you. And you'll find it fun challenging yourself on songs you've never passed before.
Relative to Rockband and other Guitar Hero games, GH3 is by far the easiest. There is a greater timing window for hitting the notes than in any other game I've played, meaning that you have more leeway for hitting your notes before it counts as a missed note. This could possibly be to balance out the dauntingly difficult expert songs, which are the only difficult aspect of the game. The note tracks are very difficult but the timing in the game is very lenient.
The campaign mode is very simplistic, requiring you to pass each song to move on to the next set of songs, with cheesy yet funny cut scenes in between (reminiscent of LEGO Indiana Jones). There are also three boss battles throughout that you must beat to progress in your career which should pose little challenge, other than possibly the final battle, which will require you to be a master of whichever difficulty you're on. Like me, you may find pleasure in increasing your cumulative career score (shown at the bottom of the song list in career.)
Graphically, the game is very nice, and a good step up from Guitar Hero 2. The note gems are shiny and colorful (who doesn't like pretty colors?) and HOPOs are more distinctly marked with a bright white halo, rather than the ambiguous HOPO representation in GH2. Very rarely will the graphics stutter when a large hoard of notes are coming up the screen and star power is activated, but I don't even remember this happening to me recently. The menus are decent looking, however some people might be turned off by the little advertisements hidden to the sides of them (such as for 7 gum). I honestly didn't notice them for months, and when I did it was more of an interesting surprise than an annoyance.
The sound of the game was also nicely done in my opinion. The sound of navigating menus seems soft and natural, as opposed to rockband, where you'll hear an almost jarring array of instrument tweaks when you're zooming through the menus. Most of the songs themselves are either the real version (aka not a cover version) or a very nicely done cover. Many seem altered slightly for the purposes of the game. For example, Rock and Roll All Nite by KISS seems to be the album version of the song until you get to the (awesome) guitar solo which seemed to have been inserted from the live version of the song.
Another large improvement from GH2 is the online multiplayer. The xbox live interface is surprisingly easy to use. You have three competitive game types available on LIVE, battle, face-off, pro face-off, as well as cooperative play if that's what you're in the mood for. And surprisingly, people still do play GH3 online, albeit only a few. Locally, the same game types are available, as well as the co-op career, which is basically the conjoined twin version of the single player career, minus the boss battles.
Bottom line: If you're an achievement completionist, don't go for this game unless you are very adept on the expert guitar, or unless you have the balls (or just uber amounds ot time) to face the challenge of making yourself better. If you're an up and coming GH master, this game will be a great stepping stone on your way to greatness. Compared to the newer Guitar Hero titles, GH3 is a worthy option. The newer titles introduced a few more guest rockstars (with a side of lawsuits), even more pristine graphics, and somewhat more strict of a timing window for hitting notes, but still very open. But if you're just looking for a fun game to play, you can get GH3 real cheap anymore; get the game and see how far it takes you.