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.
I just laughed really loud in a library at the random picture of Brian Bosworth he has there when you click on the 'Who I The Rawker?' link. People are staring at me like I killed someone. Thanks for that!