I have been a huge fan of Guitar Hero since the first second I played the original one standing in Best Buy 2 years ago.
It is now nothing to me.
Rock Band is THAT much better.
I can't really speak for any of the solo play yet, but the co-op play is some of the most fun I have ever had in local multiplayer, and even in Guitar Hero III single player isn't really all that great.
These games are definitely best played with company, and on this site, there is no doubt many of us that remember when multiplayer was all about several of us sitting around the same console.
All those great times...
Rock Band brings it all back in a very amazing way.
My son is only 8, and it is pretty tough to find games that we can both play together where we are each getting a full experience (Too complicated for him, not complicated enough for me, etc...). This was never even remotely an issue in Rock Band, because each player picks their own difficulty to play at, but there is never a feeling that they aren't able to contribute their all.
Even my 9 year old daughter, who usually hates video gaming, is able to get in on all the fun, because she happens to love Karaoke.