The streaming-music app mini-industry is interesting: I'm guessing at this point that if you use an app for music streaming, you might have found your winner already. Personally, I tend to use Slacker more than others. I've found the selection there pretty good, and the iPhone app has been challenging my music-filled iPod for listening to tunes in my car.
But I will admit that the interface for most of these music apps is lacking. It's basically a series of menus to let you look up the song you want. That's where Songza is trying to grab some users: Songza's interface is actually impressively simple and clean. And what I especially like is that if you don't have a particular song in mind to play, Songza will help you choose. Rather than going through a list of stations, the app asks what kind of mood you're in or what kind of situation you need music for and populate a playlist based on that.
I can't vouch for Songza's library at all, unfortunately, given that I've only spent about a half hour testing the service for this writeup. And the app's biggest black mark is that there's a full banner ad across the bottom of the screen -- understandable for monetization issues, but some users don't appreciate that, and there's no option to turn them off (though they do keep ads out of the audio feed, which other services don't do unless you subscribe).
Songza has also just recently released an iPad app, and it's very well done as well. If you haven't settled on a streaming music service yet or just like checking around and keeping your options open, Songza is a free download on the App Store now.