There are several members here (myself included) with XM satellite radio. You don't have to worry about it messing up the car's interior in any way.

I have the Delphi Roady2 right now. It is installed on the dash via the vent-mount kit. However, I do not like this location as it blocks one of the vents and is in a kind of strange spot.

There are several head units on the market which come "XM Ready." All you have to do is stick the antenna to the top of your car and run the line to the unit. This eliminates the need for the big box stuck to your dash and makes for a very clean looking install.