Pros: Great sound on its own
Cons: Lacks cachet, does not work well with a high end system
This little amp is a surprise. Alone it is actually not that bad with some HD650's. The sound is typical dark, liquid, and pretty nice to listen to if you're into that sort of tubey coloration.
Later, when you try to put this DAC into a revealing system, perhaps as a source for a revealing amplifier, the surprise is that the uDac 2 is actually quite terrible. And it has nothing to do with anything that has been ranted and raved about, forget the tech specs. The DAC section simply has a flat out horrid frequency response, it's boosting the upper mids in a way that makes revealing amplifiers sound ear piercingly bad... This DAC could go well driving a tubey sounding tube amp, but anything even a little revealing is going to be terrible sounding.
Build quality is decent, it is metal and the connectors are gold plated. The first version (uDac v1) had problems with channel balance on the volume pot, that seems to be largely alleviated in the uDac 2. I do not have a channel imbalance issue with my uDac 2. My uDac (v1) on the other hand was almost unusable because the channel imbalance showed up at a relatively loud volume.
Once you add up everything it does, few amps can match it in features/price, and it really does sound good on its own. But if you plan on building a system and upgrading it and hope to use the DAC section of the uDac 2 with a better amp, just DON'T, save you and your ears the trouble.