To record any kind of intstrument through USB would almost surely overwork the processor, unless it were a low volume condensor mic with low output that you could directly connect to your iPhone. They have those now, USB mics, but then you’d have to have the iPhone recognize it for what it was when it was connected to the iPhone. New challenge. Peripherals.

But as it has been said, with the right combination of the parts of GarageBand, you could put together some pretty fun little projects. May sound kind of laughable, but small groupings of midi tracks to build loops, downloaded and used as needed or erased as desired allows for a variety of sounds without wasting tons of space (instead of every guitar with effect, piano with synth, or drum); a Record To feature (is/and/or voice note); limit the files to a combination of tracks and layers; and a compression tool to blend multi tracks to a single track (taking final size down — problem, usually can’t be undone). Yeah, you would end up with a small mediocre clip that won’t play over 128kbps. But you’re no Fallout Kanya Foo Fighter pro. And remember, it’s a friggin phone!!! Remember the first nokia polyphonics that had a scale of two octaves? And they were all in the tone of a chirp.

Anyway, that gives you the software mixing tools you’d find on a simple low to mid capacity digi 4 track you’d pick up at guitar center or something but with the added benefit of having a screen to drag and drop location of loops.

Other’s are right though, capacity could become an issue. But if you’re carrying around 4 gigs of music, have a hand full of misc. crap on your phone, a couple images and a video short you downloaded from youtube, you’re still golden (as long as they’re not all playing simultaneously). If you’re carrying around multiple gigs of pics on your phone, you’re an idiot. A 2mp camera’s images aren’t big at all. You can unload them easily to your home PC (apple, win, linux, whatever) — Unless you’re using your phone as a bluetooth flashdrive to transfer your RAW file portfolio — In that case, you’re just too darn creative and need to pick one to carry around with you at every second of the day. Or use that wonderful thing called the internet, open a gmail account and email the extras to yourself. C’mon, musicians aren’t THAT lazy, freeing up space isn’t so tough.

Sounds good? It shouldn’t be for creating the perfect masterpiece. But if you get a beat in your head, some lyrics you can’t write down, or a nifty little riff you don’t want to forget, pull out your phone, spend 15 minutes, then when you get home, load it to your Mac and finish it off.

Its all about the music