I have ATH-M50S that I bought at the same time as the uDAC-2, after my initial research here on Head-Fi. I needed a closed-back headphone for public listening, and a small, USB-powered DAC that I could throw into a notebook or netbook bag. The combination sounded so good, I decided to go higher-end for a home-only set-up, and bought Sennheiser 650s at the same time as the Maverick D1, and then the A1 came close behind. Then a ton of lesser tweaks, like tubes, op-amps, USB isolators, etc., etc.
So the D1 is a very different animal than the uDAC-2; the former is not really portable and needs to be plugged into a wall outlet. I don't want to further confuse your buying options, but Audio-gd has a new unit called the NFB-12 on special promotion for $200 that merits your consideration. Use the search function here on Head-Fi for a TON of posts about it, or here is a link to the rather cryptic Chinese-English page about it from Audio-gd's own website:
http://www.audio-gd.com/Pro/Headphoneamp/NFB12/NFB12EN.htm
... and then, Maverick Audio will probably be introducing a new DAC of their own, tentatively referred to as the "D2" (subject to change) in the next several months. Personally, I am very happy with my Maverick D1 and Nuforce uDAC-2, which fill very different needs for me. The Audio-gd gear is really top-notch, though. The problem to a digital audio newbie like me is that it is a
very Chinese company, whereas Ryan Ping's Maverick Audio has a website that doesn't scare Americans like me into culture shock that they are instantly into something over their heads. Plus, there is a waiting period between payment and shipment with a lot of Audio-gd products, whereas Maverick and Nuforce fulfill orders instantly.
So, have fun deciding. Lots of good products out there, and you don't hear outright junk get talked about here on Head-Fi, which protects you from mass merchandise advertising from companies that don't offer good musical value for your dollar. And there is a thriving trade/re-sale equipment market here on the website for ongoing cyclical upgrades and hand-me-downs, so nothing is forever, except of course, death and taxes.