Zero arrived today (it shipped Tuesday and got here this afternoon, very impressed). My first visual impression was that this thing is MUCH larger than I anticipated. For whatever reason I got the idea that this thing wasn't too much bigger than a Sonic Impact T-Amp Gen2, boy was I wrong. Before hooking anything up I cracked it open to confirm the presence of the OPA627, which it did have, and that's where I ran into the first, and currently only, QC issue. The screwhole on the lower left hand corner (looking at the Zero from the front) was stripped. I had to guide the screw out with a flat head screwdriver.
As far as sound quality goes, compared to coming straight from the X-Fi, the Zero sounds just a tad bit sharper and more detailed but feels weaker (its doesn't seem as loud at a corresponding volume setting compared to the X-Fi). In short, it sounds great to me. I've only tried the headphone port once so far, and I got the impression that it didn't sound as good compared to plugging straight into the X-Fi, but then I probably need to give it some serious play time so it can settle in.
Just for the benefit of not having to sniff around the back of my computer anymore makes this an excellent buy for me. The sound quality is just a very pleasant bonus!