I've had XM Radio for nearly a year and a half... and have been hooked since Day 1. Rarely do I hear a commercial and there's always something interesting. They've announced a bunch of changes recently, one of which is a new online music service. They're going to charge $8/mo for the service, or $4/mo if you already have an XM subscription for a hardware radio. It's a cool idea, but the press releases makes it sound like you're only going to get a subset of the full channel lineup. I suppose if you've got XM in your car already, this is a good way to pay just a few dollars more and get it at home/work. I went with the Delphi radio, though, which I like because I use it in the car, at home, and in the office.
They also added XPR - XM Public Radio. I listened briefly to it earlier and can't quite tell what (if any) relationship it has to NPR (aside from a few familiar names).
Now if they'd add a weather/traffic channel for Denver/Boulder, I'd be shouting from the rooftops.