I have an XM SKyfi2, a car kit, and a Delphi boombox. Pretty convenient to pop back and forth. I installed the car kit myself in about ten minutes. Sure, it's not as pretty as a custom install, but it's not horrible either. (I hope to get my next car with it already installed.) I was concerned about the antenna, but I snaked it up through the trunk and it works beautifully.
I almost changed to Sirius because I wanted the Jimmy Buffett channel (his "radio Margaritaville") but I can listen to that online anyway, so I decided it wasn't worth the switch.
I will say this, they have messed up my billing countless times, for whatever insane reason. I'm talking bills that were triple what they should be. Everytime I have called about it, they give me three months free, and I've had free service now for about 6 months, and currently have it free until March 2007. I have been told that at that time, they'll give me a year's service for $77. I know many many people who have gotten the year's service for that price, so I believe they'll follow through on it. If not, I'll just try to cancel once again and probably end up with more free service. I can't really complain about their goofy billing when they've compensated me like that.
You might do a google search for any friend & family codes, (for whichever system you choose) I did that last year for a few gifts, and got the Delphi Skyfi2 & boombox with 3 months activation included for $59 (incredible price) and two of the Delphi Skyfi2s and car kits with 3 months activation included for $39.