Just had one installed last week. Like RedGoad, I looked at the MyFi and the Pioneer (both are XM). They are both really slick units, and I think will be in short supply for Christmas. I don't think they are IPod killers, but are a real nice compliment to the IPod.
However, one of the drawbacks for me was the biggest benefit of the unit...it is portable, and I really thought that I would accidentally leave it in the car. Since I park in the city, I know some dweeb, who wouldn't realize that the thing wouldn't work if it was stolen,would damage the car to get to it.
The second reason that I didn't buy it was the screen. Both units are about equal to the IPod screen, and I just didn't think it would be that readable at whatever speed I happened to be going.
So I bought a Sirius Starmate, that I had installed in the compartment right under the radio. It looks factory, and when you close the compartment cover, you can't see the radio...very slick.
It does use a wired FM modulator, but sounds very good (and I am in an area with not a lot of open frequencies) And pay the $75 and have someone who knows what they are doing install it. It is well worth the money. I also played with the radio settings, and turned the EG off.
Good luck.