GarageBand

Overview
In general, GarageBand is a light version of, being a loop sequencer with audio processing abilities. It excels in the quick creation of short (about 60 minutes or less) audio projects that use many splices of recorded tracks in combination with pre-recorded loops or simple MIDI synths. Its strength is the easy creation and management of many splices of audio - a relative weakness of Audacity's interface. It also makes podcast creation easy by supporting chapter creation and simple video scoring (but without video editing and support).

Amongst its weaknesses, GarageBand can only add effects to entire audio tracks at a time - a track must be split if different effects are required for different sections. Logic, for example, does not require this workround. GarageBand also lacks the ability to work with 48 kHz or greater sample-rate audio, and has on the maximum length of audio that can be exported to the lossless  format. It should be noted that although Apple refers to GarageBand exporting AIFF, it actually exports AIFF-C files, as do most applications based on QuickTime. AIFF-C files are a variant with different byte order, also uncompressed, though technically capable of containing compressed formats. AIFF-C files can cause compatibility problems with some hardware and with older audio software. Logic is one application that still exports true AIFF files.

Personal Reviews of GarageBand
CoCreatr, January 2008:

GarageBand can nicely record a speech (I did more than an hour). From that I tried to build a podcast with slides. However, I gave up trying to use GarageBand 4.1.1 to cut out the little mistakes, as the user interface is flaky for the delete and move (=join). I used Audacity for the first time and finally got some work done. No clicks on cutting into background noise, very cool. I only wish Audacity could be used like an open-reel recorder, that is listen to what is at the playhead while cueing back and forth, even if pitch change is unavoidable.

Arnebab, 2006:

You will experience some nagging inabilities, but it is a good deal more efficient for the creative process. Limitations: No Rate-Changes (always 44.1kHz), only about 1h of recording possible (stumbled into this when trying to record a longer session), uses quite much CPU and when you have other tasks running it often stops recording (with errors like "disk too slow" or "too many tracks"), where Audacity goes on recording. But except from these it is really worth the price, especially because of the real-time effects and presets (like echo and reverb and some more presets for "live"-sounding voice, rock-guitar, etc.). But you won't be able to ever go on from the Mac to other Systems, if you want to stick to GaragBand for a longer time.

Dominic Mazzoni, January 2004:

GarageBand is very much like all of Apple's other iLife applications: very professional looking, incredibly intuitive interface, with just about every feature that a beginner would need but no more advanced power-user controls.

GarageBand definitely fits into the category of an integrated audio/midi app, so you shouldn't really think of it as a midi sequencer or an audio editor, since it's not really either. Here are three different ways one could use it:

1: Loops: GarageBand comes with hundreds of built-in drum loops, bass lines, guitar licks and so on that are very easy to assemble to create a song just by clicking and dragging. It's hard to describe just how fun and easy the interface is. It makes it totally effortless. Besides the built-in loops, of course, it will import any AIFF, MP3 or MIDI file that you can use as a loop.

Biggest limitation: Every song is in only one key. MIDI loops are transposed from their original key into the key of your song. So it's impossible to use it to create a Karaoke background for a popular song just by typing in the chords; you have to record the parts yourself if you want to do this. Definitely aimed at beginners and non-musicians, but it's a bummer, because just by adding the ability to switch keys, you could totally create just about any chord changes. With GarageBand versions 3 and 4 you can transpose selected regions and get around this limitation.

2: MIDI sequencing: The software instruments are decent. Way better than a SoundBlaster, better than QuickTime musical instruments - but nowhere near as good as my Roland XP-30 keyboard (which is five years old already). Also the selection is limited - mostly guitars, keyboards, drums, and the bare minimum of other backgrounds you'd need for pop music. No solo string instruments, solo trumpets, or anything like that (need to buy or find free or  licensed instruments). Still, I was impressed with the responsiveness - I've seen many software instruments lag too far behind the MIDI events - and the quality is good enough that it's fun.

Very easy to record MIDI tracks, and the UI is great. Unfortunately there's very limited editing, nowhere near enough for someone who's used to fine-tuning MIDI sequences. I couldn't even figure out how to create a new note, so I don't think I could use it to sequence drum patterns by hand; I'd have to record it on my keyboard slowly.

3: Audio recording: These are called "real instruments" as opposed to software instruments. Again, very easy to use and seems to work well.

Biggest strength: built-in effects, all real-time. You choose the effects once for each track, with no automation possible. Built-in effects are organized by the type of instrument they should be used for, like enhancing a male voice, or creating an electric guitar sound from a guitar pickup.

Weaknesses: Limited to 16 bit/44100 Hz (Garage Band 4 can do 24 bit). Seems to be able to resample on the fly so you can import other formats, but you only get 16 bit/44100 Hz quality. Almost no editing at any level other than the track level. No competition for Audacity here!

Anyway, it's too bad about a few of the limitations in particular, but all of the other limitations make sense given what it's trying to be, and I would totally recommend it in a second for its intended market, i.e. Garage Bands. Audacity should complement it well for anyone who needs to hand-edit any of their individual parts or wants any of Audacity's effects.

Some comparisons of GarageBand and Audacity
GarageBand is great, but its strengths and Audacity's are very different. If you're into working with loops, or if you're interested in any of the other iLife upgrades (iPhoto is a huge upgrade, and iDVD is improved quite a bit, too) then it's well worth getting Garage Band. For everyone else, either use Audacity for audio and for MIDI, or else upgrade to an expensive professional package like Logic Audio that does all of this and more.