I love Pandora and TuneWiki, but never got into Last.fm much. I wasn't impressed with Slacker. Though TuneWiki is able to play shoutcast stations, the streaming almost never works for my favorite streaming site Somafm.com. I use DroidLive and it streams all my stations from Soma exellently. My favorites are Groove Salad, Beat Blender, and PopTron. Pandora's database is only 300,000 songs from what I've heard, and it can get repetitive. I sound like a shill for Somafm but it's truly great commercial free internet radio that I stream 24/7 on my home computer, and after trying 5 or 6 streaming apps for Android, DroidLive does the best job and has a nice interface.
I really hope TuneWiki and Pandora make their buttons larger and more responsive though...I really can't understand why their interfaces are so terrible.