I think it's only worth it if you spend a lot of time in the car or go on frequent long trips. For everyday commuting I wouldn't keep mine, but since I drive back to Tennessee a lot, I think it's totally worth it.
The monthly cost isn't that bad depending on your package. The most expensive one is XM with the Best of Sirius for $17/mo. The only reason I might pay for that is to be able to listen to any NFL game, because it's Sundays that I'm frequently driving back from TN. But I do the XM everything that has music and sports (MLB, NHL, and College Football and Basketball) for $13/mo. The biggest cost is up front with installation and the cost of a new radio if your car doesn't already come with it.