Chris,
GarageBand for me as a musician is one of the most astonishing entry level music software existing. It is more than sufficiant for simple arrangement and composing projects. Considering the time and money I spent about 15 - 20 years ago to be able to have a (not even) comparable setup, GarageBand blows me away. In 1990 I bought my first computer, Atari 1040 ST and the predecessor of Logic which was then called Notator developed by the small company C-Lab located in Hamburg, Germany for about 2000 $. The hardware I then bought over the next few years such as sound modules, synthesizers, mixers, effect boards etc. was easily more then 10,000 $.
But I had no audio recording, and the midrange mixer and effects which I had produced quite a bit of noise. From this perspective Garageband is a full digital recording studio with more effects, sounds and audio capabilities than I ever could afford 15 - 20 years back.
I even use GarageBand as a sound module (software instruments) for my live performances.
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Although a large part of its market is people who don’t play an intsrument, it sadly lacks in sufficient loops to create anything complete without a lot of hard work.
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GarageBand was definitly made for musicians. As you said it might be fun for a while for people who don't play an instrument but then gets boring. GarageBand was not even made for DJ's and the like who create music with a very different approach.
Magic GarageBand now is a very differnt story it is definitly a toy made mainly for people who don't play an instrument.
Overall I would say that for non musicians GarageBand can't be much more than a toy most of the time but for musicians it is a small yet quite powerfull easy to use tool usefull and very welcome especially for no extra charge when bying a Mac.
iLife '08 - Part 3: iWeb, GarageBand, and iDVD