It's a fun toy. But it cannot be remotely considered serious music-making software for a number of reasons:
(1) It is buggy. I've used it a lot, and every couple of hours it will hard crash on me. It's also caused my iPad 2 to completely lock up, necessitating a hard reboot. And it stutters, sometimes badly, with cracking and popping when attempting playback.
(2) You can't change time signatures. You want to do a waltz tempo in 2/3? Too bad. A ballad in 3/4? Nope. You are stuck with 4/4. Which is pretty damn lame.
(3) No editing individual note data. You've recorded a drum loop and want to duplicate it but swap out the high hat for the ride ( something that happens in nearly every song)? Too bad, rerecord the whole thing. Want a drum fill right before the end of a 16 bar section? Pretty much impossible. Lame.
(4) Touch-velocity sucks hard, and there's no way to disable it. It is entirely unpredictable, thus, one out of 3 snare hits will be absurdly loud, while one will be almost inaudible. Or a random piano chord will be absurdly loud. You get the picture. If one could disable touch-velocity, or edit individual note data (see No. 3, above), this would not be an issue. But you can't, so it is.
(4) No changing or adding chords on smart guitar. Given their affinity for augmentations, that pretty much excludes any Beatles song from ever being created with GB. I wanna hold your hand? Sorry, it has a major 6 and a major 3 chord in the verses, and so it can't be done with GB. And so it goes for 1/2 of all hit songs ever made.
(5) You can't change keys.
Don't get me wrong, it's a fun piece of software to putz around on. But these are major limitations that need to be addressed. Until they are, nothing remotely serious can be attempted on GB for iPad.
There is also a very long review on Ars (which curiously omits the above shortcomings):
http://arstechnica.com/apple/review...ipad-a-killer-app-for-budding-songwriters.ars