I've used it, although it's been awhile. It seemed to me that all the services have mostly the same music, and then some is missing from all (independent or small label CDs, the Beatles). I think whoever decides, whether it's the musicians or their label, it's usually across the board. But the interfaces vary a lot. I really like Rhapsody--you should check that out. I found it much easier to control than Yahoo!'s (and I was pretty pro-Yahoo! at the time I tried Unlimited, because I worked for them then). The availability of jazz albums on Rhapsody is amazing (but, as I said, I'm pretty sure it's almost the same on all web-based music streaming services). I would do the free trials (if they have them), and decide based on how well you like the user interface (e.g., how easy it is to find what you want, to make your own play lists, etc.). Rhapsody is easiest for me to use, but admittedly, that's partly because I'm most familiar with that one. I feel like I have nearly the entire history of recorded jazz available instantly to listen to or play along with.
The jazz coverage is good, but all the services have major gaps in certain genres. I've noticed that especially in blues and celtic music. The coverage in classical music is good in terms of composers and works, but not in terms of performers/performances, which is kind of a big deal in classical music. I mean, you might *want* to hear Beethoven's 5th Piano Concerto by a certain pianist and not find it. Also, as with Amazon, All Music Guide, and other online services, the search functionality for classical music SUCKS!!! It is admittedly a complicated programming problem, since there are composers, works, conductors, orchestras, solists, specific sessions, etc. (Janos Starker recorded the Bach Cello Suites five times! and I think Toscanini and Horowitz recorded Tchaikovsky's First Piano Concerto a couple of times, at least). But these search engines seem to have been devised by engineers who know nada about classical music.