XM opinion
I have one of the Alpine head units with iPod integration. It came satellite ready (you could choose either the Sirius or XM module and antenna).
I decided to get the XM. I was able to lock in a 2 year commitment prior to them raising their prices, so I'm only paying like 8/month right now.
We put it in the "family" car (Subaru Forester), and I absolutely love it. The audio quality, while not CD quality, is still far better than FM.
The biggest thing about it, however, is the content. Great news, sports, and most importantly music. I'm a big jazz fan (classic "real" jazz, not the easy listening garbage), and it's the only game in town.
In fact, in my area, terestial radio is basically worthless unless you want Clear Channel shoving the same classic rock, country, and easy listening down your throat daily.
So, yeah, satellite radio is a good thing; pretty much like cable or directv -vs- over the air networks. You get what you pay for, but if you're not one who drives a lot or isn't into music, than it probably wouldn't be your thing.