I have a duo, that I'll mention briefly. I use it only for my laptop, and only for field recording and personal demos. My studio DAW is setup with
a DM-24 going to a Motu mkIII (which is a dream)
The duo is a really nice swiss army type piece of gear. The pres on it are fairly clean, and have used it as a standalone for this. I previously had a presonus blue tube, and while it's a little bit apples and organges (solid state vs. Tube Pre) the duo is just plain cleaner.
As for the usb side of things, it works at 24bit 48k, with an unnoticable amount of latency, and for my uses (live and playtime) it's no problem, now bump it up to 24/96 and you do have to do a little tweaking, but it's not unreasonable.
Like I'm sure a lot of people will say on this board, USB isn't ideal for audio.
That said, if you got a duo, you could use it in conjunction with one of m-audio's duo 2496 cards, and imrpove performance.
That way, you could build your studio bit by bit.
Overall, I've been happy with the duo, and would feel comfortable recommending it as a piece of gear that will be able to do the job, and not become useless when you upgrade.
I haven't heard the new tascam, but my only advice with usb is to keep it simple (the 224 usb interface with faders etc... was aweful to use)
Regards,
Rich