Since it's so expensive, you'd expect Jam Pack to act like a sixth iLife component, but the program merely gives GarageBand a shot in the arm. The GarageBand loop browser still offers the same options, but Jam Pack lengthens the loop list. Similarly, instead of entirely new software instruments, you get more menu choices for the existing ones. For example, Baritone and Tenor join the Solo sax selection.
If you're an effects-hungry guitarist, you might have fun with some of Jam Pack's effects and amp simulators. Fifteen new options, from Bright Country to Eighties Pop, spice up things, but they just aren't rockin' enough to make us spend 100 bucks.
If you're looking for a greater variety of sounds and instruments, you might do better buying Apple/M-Audio's $99 USB MIDI keyboard, the Keystation 49e. It lets you record original tracks and loops using the software instruments already in GarageBand.
Guitarists might appreciate Jam Pack's 15 new amp sounds, but there's not much else here.