I took Ambiance Sun (The Big Purple Bus) from Ft. Lauderdale to Key West back in January. It was great - they had an on-board attendant that actually fed us and was really pleasant. The bus service has movies and all kinds of extras that don't cost anything more than the ticket. I think I paid like 70 bucks each way, but it was well worth it.

I just checked out their web site and they've changed their name to TransFloridian. (www.translforidian.com)
It looks like they have even more service than before.