I'm using the Duet, it's a treat. If you don't need spdif/optical i/o, it's indeed the only soundcard in town for the money (hi Paul!).
First I had some glitches with VI when I tried to use the Duet with a first gen MBP (17"/2.16gHz), turns out it was a RAM problem (2GB only). Works perfectly with the latest MacPro loaded with +4GB of RAM under Leopard.
Christian
EDIT. The Duet is plugged to a power amp feeding Tannoys LGM and PBM6.5II. No problem.