I have XM and I'll say it's awesome. The antenna, radio, and head unit are essential unless you have an XM or Sirius ready head unit. I know this sounds confusing, and to be honest it confused the hell out of me.
As far as installation, I have a pickup truck. I put the magnetic antenna on top, ran the wire from the tail light on the roof to the inside of the truck, routed the wire under the carpet and found a nice spot to hide the radio. If you're not comfortable with removing some interior paneling and lifting up the carpet and generally poking around in your vehicle, have it professionally installed.
It sounds awesome through my sterio - much better than FM.
Interruptions in service shocked the hell out of me the first time; I thought I had fucked up something in the install. Sometimes if you're going under trees and such (you can't see the sky very well) it won't pick up. It's not supposed to work in garages but it works fine in mine. That still baffles the hell out of me. When it does cut out, it's usually under a second and the "white noise" isn't terrible.
I'd recommend the hell out of it.
PM me if you have any more questions.