Rock Band 3 is brilliant and has finally overtaken Guitar Hero.
The good:
It's an extremely good looking game, the characters are all very good looking and even the fretboard and the notes look good.
The setlist is pretty big, with over 80 songs and over 1000 downloadable songs, you can basically get any genre of music that you like, and it's pretty much guaranteed that you'll be able to download at least 10 songs that you love.
The game is very long, we're talking like 20 ish hours to complete most of it, but then you get multiplayer and just general quick play to play all your favourite songs.
You get to choose your own setlists on 'career' mode, this is an amazing thing that Guitar Hero doesn't do, you get a setlist on Guitar Hero and you basically have to do it, or you can't progress, whereas you get options to an extent on Rock Band, you don't always get the songs you want, but if you buy something like 3 Disturbed songs, you could get the option to have a Disturbed setlist and it'll pick those 3 songs for you. It's very good.
It's extremely addictive.
The bad:
The notes are harder to hit on Rock Band than on Guitar Hero, I've always thought that, because they're smaller, and also, it's not as fast as Guitar Hero, which makes it difficult to adapt to.
Songs are expensive. I really don't like how much they cost and I would prefer if they were cheaper, it's 160 microsoft points per song, may not sound like much, but it starts to add up when you're buying 5+ songs.
Can be very difficult on faster songs, which is probably because I can't adapt to it after hundreds of hours on Guitar Hero.
Overall:
Rock Band 3 is the best music game out there, with the best selection of songs with the best graphics. It's brilliant.