TheFeed loves Rock Band, but we do not like the guitar that comes with it. While our problems are subtle (the "action" on the strum bar is sludgy and weak, and the thing just feels flimsy), other people's problems are much more concrete: Namely reports are coming in from all over the internet of strum bars that stop working after a couple hours of play, and axes that register random strokes, making Rock Band unplayable on guitar.
Yes, there are fixes, but they involve taking your guitar apart and void your warranty. But have faith: Harmonix, the game's manufacturer, will send you a new guitar if you need it, as the game is covered by warranty. Go here for details.
Just click here and get ready to go rockless for a few weeks while you wait for your new axe...or just buy a Guitar Hero 3 guitar. They come out in early 2008, and they're wireless.
EDIT: You can't currently use the GH 3 guitar with the PlayStation 3 version of Rock Band. Thanks, posters!