I've been using the Tascam US-428 with Cakewalk Pro Audio 9, and now Sonar on a PC. This combo works well at 16 bit, although I've had problems with 24 bit (not working). You get 4 audio streams in and 2 out, and 2 midi ports, sounds good, pretty good bang for the buck ratio, IMO.

I'm with Doc on that one...I'm using the 428 with 24 bit mode into Nuendo and overall it ROCKS!! solid streaming audio thru USB..the fact that you have a controller surface as well makes it a superb instrument for DAW recording..hope that helps
Opus

Well, Doc and Opus have done yeoman work on the Tascam forum in getting the US-438 working and defending its, uh, virtues.

As Doc noted, the 24-bit WDM drivers don't work yet, but as a digital I/O and MIDI interace it works fine.

Nevertheless, I don't wax quite as enthusiastically, since a control surface has not been coming for Cakewalk (even with the good Doc's work). That is supposed to be forthcoming in the Sonar upgrade in about a month, but who knows..?

That said, I just got a VM3100 Pro for $400 and an RPC-1 for $230. Eight tracks in and out, with great effects (in addition to SONAR's), with M-Audio producing the card and drivers.

And with that kind of system (which already interfaces beautifully as a control surface with SONAR), the Blue Amoeba is going to be sitting under a desk, acting as only a digital I/O for my laptop.

Unless, of course, some one wants to make me an offer I can't refuse..... :p