I have Sirius ('04 with the Sirius-ready Audiophile head unit), and I love it; most of the time I'm in the car, I'm listening to it (Radio Margaritaville, baby :Beer ). I don't remember if it was in '03 or '04, but Ford switched to Sirius-ready units in the LS, which only require the installation of the Ford Sirius adapter and antenna. It's completely seamless (it uses the radio display), and the steering wheel controls will operate the Sirius unit.
For the earlier models, you'll have to go with another unit; in the Crutchfield catalog, they had a neat looking unit with a low-profile display. Dunno if they use FM modulators or hook into the auxiliary inputs of the head unit, though. Seems like I read somewhere about an adapter (from PIE?) that will alow you to connect an aftermarket Sirius adapter to the auxiliary RCA inputs on the factory head unit.