my experience couldn't be any more different...
pre-xm, i would mostly find new music by downloading/streaming on the internet or attending shows that seemed cool. i usually work at home and listen to music 24/7.
i've had it now for about a year and virtually every CD I've bought since then (a few dozen) were things that I first heard on XM, and which probably haven't been played on HFS or sirius or anywhere else, at least not very much...
these include CDs by bent, baxter, lemon jelly, i am the WTC, royksopp, soliders of jah, scotty...
of course, there have been others that I might have found somewhere else, like the thrills, alot of stuff I *didn't* buy because I heard it on XM and it wasn't worth it, and lots of stuff that I already liked and knew about, but which is nonetheless not reliably heard anywhere else.
then there's the channels of stuff that are fun to listen to once in a while even though i'd be unlikely to ever buy CDs in the genre, like on the rocks (76) or boneyard (41).