I'll kick this off - for me, Guitar Hero 3 takes a step away from real music towards video game, which depending on your point of view either rocks or defeats the whole purpose. However, it captures the Guitar Hero spirit. It's a good game (yet does not eclipse GH2). Harmonix's spirit is almost entirely gone from the game, with Neversoft imprinting their style on everything - characters, graphics, menus, loading screens, all of it. I'm over it. I don't care anymore. The game's story won me over with it's crazy scope. I haven't touch the boss battles yet (playing co-op career) so I can't comment on them.
For a game that focuses so much attention on the "notes" I thought GH3 was more gamey and less musicy. I'm not a pro-guitarist, but I'll lay money down that GH3's fingering isn't as "accurate" as GH2's. I wonder if I spend a lot of time in this game that I will lose my GH2 skills. However, the Hammer-Ons, Pull-Offs are a ton more fun, and the things the game does with the HOPOs is absolutely crazy. Oh, and the wammy effects are awesome! Real sound processing and not just frequency modulation.
There are some awesome songs and some duds, although the duds surprised me as they were usually the most famous songs. I wasn't impressed by Sabatoge, for example. Meh. Barracuda? Let down. Welcome to the Jungle is epic but can't match Sweet Child of Mine from GH2. However, songs that blew me away included: Reptilla, My Name is Jonah, The Seeker, Suck My Kiss, Bulls on Parade, One (One-mutha-f*ckin-One, muthf*cker!), Black Magic Woman, 3s & 7s, Helicopter (that one came out of nowhere). I even clicked with The Metal after I absolutely hated it in the demo. As I listen to my roommate play, there are a lot more songs I'm hearing that I want to dive into. That's a lot of songs to like. I don't like the new mix of sound levels, guitar, mistakes, and crowd are soft, everything else is too loud.
Shout out to polypusher - you were right, Cities on Flame with Rock and Roll is the best song on the disc.
The guitar is one of the best you can buy. I highly recommend getting it if you plan on spending any time with this game. Some people have said it's a better guitar than Rock Band's ... they just might be right. I haven't played with RB's controller but GH3's is the best one I've touched so far. God Bless competition.
If you can't tell, the game is warming up to me. Change is difficult, but this is an excellent start for Neversoft.