Were you looking at Sirius or XM? You can test-drive both (at least the music channels) online through streaming for 3 days for free. I do this at work all the time with rotating email addresses (part of the fun of controlling my own domain and mail accounts). My thoughts on them:
SIRIUS:
Good all around service, 3 satellites in a figure-8 orbit so they are constantly moving. Plays more popular music more often than XM. Quite a large variety of genres, especially within the Rock category. $12.95 per month, which includes free Internet streaming. Over 120 channels, all music channels are "commercial free" but have DJs breaking in quite often (similar to FM radio). Has contract with NFL for games.
XM:
Good service also, 2 satellites ("Rock" and "Roll") that are in geosynchronous orbit over US. One East coast, one West coast. Plays a lot of popular music, but with more time between repeat songs than Sirius, there are a TON of songs that are rare and not heard often. I hear many songs I'd completely forgotten existed. Over 150 channels. No commercials on music channels, and very little DJ chatter, usually once every 5 songs or so to say what station you're on or to tell you the number to call in for requests. $12.95 a month like Sirius, web streaming is now included with basic service. You can get web streaming ONLY (no regular XM tuner) for $7.99. There are discounts for "contract service" if you pre-order for 3+ years its down to $9.99/month effectively. They also have additional radios for $6.99 a month for multiple car/home/mobile tuners. XM has ~8 MLB channels so you never miss a game but doesn't have other sports other than ESPN Radio and Fox Sports.
I personally have XM in my Tahoe, and I like it a lot. Long trips are so much better when you don't have to find a new station every 50-100 miles (if you're lucky). The local weather/traffic stations were invaluble on my trip to Washington D.C. last summer considering I arrived in rush hour and wasn't familiar with the roads at all. The only downfall (which affects Sirius as well) is tall buildings, like in a large city downtown area, where there are no terrestrial backup towers as the buildings block the signal (and I work in downtown Des Moines, IA so I get cut off as soon as I hit a building more than 5 stories to my immediate south side). Larger cities have terrestrial towers to negate this problem.