I really like my skyfi unit. If you're a music nut and drive a lot it's a good purchase. Think of it as cable tv for your radio, 150-some channels. The skyfi is a 2nd generation receiver and has a large text display. Many automotive companies are starting to integrate XM into the OEM systems (e.g. Honda, GM).
The sound quality (a digital signal) is inbetween FM and CD. I get fuzz under power lines and bridges, but for only a moment or so.
The monthly cost is $10/month and there is no contract. I think there's a $15 activation fee, though.