I've had mine for over a month and love it. You have to purchase two hardware pieces, the antenna and the decoder, about $250 total. Then you pay for the subscription, you can sign up at sirius.com, it's $12.95 a month. No commercials is a beautiful, beautiful thing. The sound is great, although it will pause for second if I go under an overpass or through a tunnel, but it's great otherwise.
I had mine installed at my local Tweeter center, they mounted my antenna on the rear deck instead of the roof, it gets great reception there, and doesn't mar the appearance of my pumpkin

The decoder is in the trunk, behind the side-liner on the drivers side, there is a ton of room back there, and it's well ventilated. I drive around all the time for my job, going from client to client, so I love my Sirius radio. I suggest you go sirius.com and check out the streams they've got, see if they sound interesting to you. I think you can even preview them off the website, if you like it, go for it, it's great.
PS: this does not disable your normal FM/AM tuner, Sirius just becomes a third option when you press SRC button on our head-units.