Jonas

There's a pretty elegant OSX solution for using ANY USB-based controlled for musical purposes. The program is called JunXion, it's made by a foundation in the Netherlands called STEIM (http://www.steim.org/steim/junxion_v3.html).
JunXion actually lets you map the controls on USB devices directly to MIDI, so you could theoretically use your X-box controller to operate an external synthesizer if you so desired.
The only down side is that it costs money. STEIM is a non-profit electronic music research lab. Though if you're really into this sort of stuff it's a worthwhile investment - yesterday I used a wacom drawing tablet to control the envelop parameters on a synth.