That's a good question. I'm looking for a new usb dac myself and can't for the life of me make up my mind.
I'm thinking for $400 the headroom micro dac is probably a safe bet, but personally, I'm thinking of waiting and saving up for the ultra micro dac.
Other dacs in the running: Emu 0404 usb (under $200), Fubar (WAY under $200), Keces usb dac for sale on ebay (waiting to hear more reviews about this one).
The more I wait though, the more I may just save and invest...