This is a new application available on the iphone by Matthew Benney. Finally a great set of drum tracks for guitar players. You've got a range of styles (Rock, Blues, Reggae, Metal, Country...) as well as different times (4/4, 3/4 etc) and speeds. Think of this as the best most fun metronome application you've ever had. If you get bored with a standard metronome, this will get you more energized!
The only limitations to BackBeats are those inherent to any iPhone music app. And that is, you can't have BackBeats playing it's drum tracks in the background while using a separate iPhone app such as AmpKit or Amplitube as your guitar modeling application.
The long and short of that means that as useful as BackBeats is, you still need some other way to amplify your guitar. If your amplifier has an input jack for an MP3 player, then that's a great solution. Otherwise, I'm hoping that Matthew Benney can figure out a way to make his application play in the background. Still, for under a buck, this is a great application and if you play guitar or bass, you should pick it up.