The differences between tiers - Unlimited is better quality, has an offline mode, and allows full cell phone access. And you need to sign up for an invite to access the free version - which I've done, so we'll see.
About a year ago, I caught them playing "Rock Star" on Letterman. It was pretty kickass- lots of big guitars and rock. Lately, I've tried to download the song at Kazaa, but it seems different- the guitars are replaced by beats.