Fellow Keyboard contributor Francis Preve talks GarageBand in a MacJams interview.
Francis has a great take on professionals using GarageBand; he himself
is a virtuoso with Apple Loops, and the Apple mania has spread as far
as Ed from Barenaked Ladies. You can open up the GB instruments and
your GB files in Logic Express 7 and Logic Pro 7, incidentally; it
works like a charm.

(Thanks for the link, Fran — though I'm still working on how to
respond to the glamour shot by the pool. Maybe I can remix on the C
subway train, a la M-Audio's ad campaign?)