I sold my Duet (1), and now have an Mbox 3 (standard USB).
Advantage of Mbox:
Direct monitoring w/ Reverb/Delay in Cue Mix
Built in tuner app
Separate knobs for input and monitor levels
No breakout cable
Works with Mac or Windows OS
Price (I paid $379 for my Mbox on Amazon w/ the 15% off deal they have right now).
I use it with my Macbook Pro 2.53 Core 2 Duo with PT10HD (I have and HD system as well). I regularly use 32 sample buffer for very low latency using Eleven, and a bunch of Avid instruments. I don't miss the Duet at all.