Apple's release of GarageBand for iPad has, in a small way, rocked my world. There have been a few admirable attempts at multi-track recorders, but this US$4.99 app from Apple takes the cake. The best part is that it's not just a musicians tool; anybody with half an ear for a tune can start composing great-sounding tracks from the moment they launch the app.
When I was a kid, I had a Tascam 4-track, an acoustic guitar and a general MIDI synth. I spent hours, even days, at a time playing with them. If I'd had this back then, I might never have left the house. With an array of built-in instruments and an eight-track recorder, you don't even need to plug anything in to start rocking. That's not to say you can't, I've had decent luck plugging in a Blue Snowball, a couple of M-Audio keyboards and my electric guitar via the iRig.
The "Smart" instruments included in GarageBand can build backing tracks in minutes, with a pretty good selection of pianos, organs, guitars (electric and acoustic), basses and drums. You can record in sections, pattern style, or you can record straight through. In section mode, most instruments will stop recording and start playback at the end of a section, but drums, handily enough, automatically overdub to let you build multiple layers of percussion with ease.
The pressure sensitivity that was demoed at the iPad 2 event is what I would label "A for effort." It uses the accelerometer to determine the velocity of the key hit, but I couldn't make it consistent no matter how many surfaces and positions I tried it in. It's cool, but if you have an external controller, you're way better off with that. As a side note, you can get quite a few USB peripherals working via the Camera Connection Kit if you use a powered USB hub in the middle. That stumped me for a while; even powered keyboards were rejected as requiring too much power to operate.
The iRig setup with the built-in guitar amps didn't blow me away. That's not to say that the Guitar Amp section of GarageBand is lacking -- it's not. I just need a better input, I think. GarageBand inputs have a noise gate, but the noise bed on the iRig was high enough that the cuts in and out were too abrupt to be useful. I'm very much looking forward to seeing what I can do with Apogee's Jam guitar interface when it comes out.
The MIDI control is great for note on/off, but you won't get a lot out of any extra knobs and buttons you have, at least beyond pitch bend and modulation. I don't see that as a major handicap, though. If you look at GarageBand on the iPad the way I look at GarageBand on the Mac, it's an amazing tool for "music sketches," and it provides almost everything you need to make a polished final piece.
I'd love to take my recordings from GarageBand on my iPad and edit them further on my Mac, either in GarageBand or Logic. This isn't currently possible, but Apple has promised a Mac update that can read the iPad tracks soon. Regardless, for five bucks, you can't really beat this app, whether you're having fun or making some serious jams. Check it out on the App Store.