It seems only fitting that a program created for angst-ridden aspiring musician types to record and edit their latest effort, would be created for the Mac. In its latest iteration GarageBand doesn't even require a pre-requisite level of skill as Apple has integrated lessons into the software.
The built-in tutorials cater for the basics of guitar and piano up to, say, Oasis levels of complexity. Aside from the guitar teacher annoying me for reasons I'm not quite able to put my finger on - there's just something not quite right about the guy - the tutorials are pretty good.
Video is presented in high definition, which is helpful for picking out exactly what fret or key is being fingered, and a variety of camera angles are available. Music can be presented in chords and tabs depending on preference. A metronome is available - pre-configured in-tempo with the current lesson - if desired. Best of all, music can be slowed down without messing with its pitch, making trickier lessons that bit easier.
If these basic lessons are below you - I still struggle with even the un-chorded Smoke on the Water, but can tinkle a pretty decent rendition of Feeling Good so the piano lessons seemed a little pointless - then there are advanced lessons to download, as a small fee, offering more in-depth guidance.
If even advanced lessons aren't quite enough then artist lessons will be just the thing. For a few pounds a piece, Apple offers a selection of downloadable tutorials with numerous musicians teaching their own songs, interspersed with a little background information about the tracks and the occasional irrelevant anecdote. Alas Fall Out Boy's Patrick Stump was unable to coach anything resembling an accurate rendition of I Don't Care - the guitar is a silly instrument - but under Norah Jones guidance I was able to pick up Thinking About You. Which is to say: artist lessons are pretty neat.
If you just want to edit or create music GarageBand still caters for that need. If you want to use a Mac as a substitute for a physical amp, then you might be pleased to know a range of amp effects are built into GarageBand '09, in addition to a bunch of stompbox effects. There's no getting away from the fact that these sound like what they are - generated - but they're good enough for practising.