Personally, I'm using a Behringer UCG102, which I had heard worked well with the Pi. It was not doing duplex audio, however, until I changed the samplerate from 44100 down to 32000 (honestly, I don't really miss hearing that last couple thousand hertz). 

It seems like whatever device you have, you need to fiddle around with it a lot before it works at all.

Good luck,

-Brian Fay