Pandora is OK, and I do pay $36 annually. My beef is that their music isn't all that varied. Every time I put in an artist, the similar ones that come up I'm already well-familiar with. I mean come on, is there anyone who is a serious Allman Brothers Band fan who isn't already familiar with Warren Haynes' or Derek Trucks' musical output? I can't remember the last time a band was played which was new to me. Nonetheless, it is a nice convenience to have on my smartphone and my roku rig at home.
I've not checked out spotify or slacker radio, and probably never will. I did, however, pay for the TuneIn premium app from the android store and have access to a ton of good stuff, much more than just music. Lots of BBC stuff, old time radio stuff, and radio streams from all over the world.
For serious listening, it will always be on CD or vinyl at home in my listening room.
Edit: I've had SIRIUS for 6 months free that came with a car I purchased, but the programming is terrible. Didn't continue it, and will never purchase it. Satellite radio days are numbered. Once Howard Stern leaves, so does the audience. The "radio" model in automobiles will change; we'll all have 4g or faster data plans on our car, and "radio" will be a series of apps on a touch panel.