I have XM radio and have had before they combined with Sirius. I have noticed that here on the Right coast I receive intermittent signal in dense forest and sometimes around power lines (for example as in when I am driving from point A to point B and power lines parallel the road). This is less a problem when in camp.
I like the XM a lot but listen to only a few stations. I like it enough that I purchased extra car kits along with the receiver. That way I can just move the receiver from car to car. At home I listen to Pandora, but if your camping a lot that requires cell signal and starts to eat into the amount of data you can download.