If you are thinking about satellite radio in general, you may also want to check out sirius.
Do you already have an aftermarket satellite ready deck in your car? If so, I would say go with whatever service the deck controls.
If you are looking for one of the plug and play receivers, check out best buy, circuit city, etc.
You can also check out xmradio.com and sirius.com for any special deals. If you buy a sirius ready radio(clarion, kenwood, panasonic, etc), you get the sirius receiver free when you sign up for a year(and when you sign up for a year, you get 3 months free).
I love sirius, and I think once you have satellite radio(either service), you will never go without it again. :O)