I've looked at a lot
of these, and none of them quite meet all my criteria. I'm also an avid XM listener -- and have been pretty much since inception. [The Delphi XM Roadie was state of the art in 2003.] I can easily move to XM on demand and other streaming services for music while getting a cheap navigation upgrade. I've already got a Kenwood DDX-514, so I need to keep a lot of the functionality, but I'm not going to press the button a $400 toy until I get the last couple features I want.
I can easily tether my phone to provide streaming data, but I'd probably also end up moving to either an account and external data for the unit itself, or I'd just permanently plant a hotspot of some sort in the car. There's a few per-byte no-monthly providers of hotspots, but they're all 3G right now. $45 gets you three gigs with no end date. http://www.internet-go.com/
Pandora, for example, takes 35 megs per hour to listen to. That's only 85 hours of Pandora. XM Online could be as much as 60 megs/hour, meaning only 50 hours. If you're not a heavy "live" listener, you'd benefit on prepaid gigs, but a monthly plan might work better for higher data usage.