I agree - Matias' recordings sound very accurate to me as well. Nice job!
So far, I'd say my experience with my Quiet Pro is pretty much in line with the Engadget review. I'm quite pleased with the compromises overall. I find the tactility of the Quiet Pro's switches to actually be quite good. To me, the compromise seems to be more about slightly reduced key travel, similar to the feel of putting rubber O-rings on Cherry switches to quiet down the "clack" when bottoming out. However, the way the dampening "bumpers" on the Quiet Pro work definitely has a less noticeable effect than the O-ring solution. There's more "give" when bottoming out, so I'd say if you're a heavy-handed typist like me, the very bottom of the keypress feels similar to a high quality rubber dome "landing". (The tactile point, of course, feels NOTHING like a rubber dome, just to be clear...that part definitely still has the ALPS spirit intact!)
When comparing the tactility vs. sound tradeoff against my Cherry Brown boards, the Quiet Pro wins, no contest, largely because I do tend to type fairly hard, so for me Brown switches have very little tactility and actually "clack" pretty noisily. If you have a very light touch, then I think the sound difference on the Browns would be less noticeable (and maybe the tactility would be sufficient), but in my experience it is VERY difficult not to bottom out when typing on either Brown or Red Cherry switches.
Regarding build quality, I'd rank the Quiet Pro (glossy black PC version) slightly below Das Keyboard and Filco overall, mainly because the plastic casing doesn't feel quite as sturdy or premium and neither the flip-out feet nor the back edge of the Quiet Pro have anti-skid rubber on them. It also feels slightly lighter weight overall, though admittedly I haven't taken the time to actually put it on the scale yet. That said, I haven't noticed any pinging or creaking / flexing problems, so the Quiet Pro is certainly sturdy enough in a practical sense. One thing I am impressed by build-wise is how smooth and rattle-free the space bar and other stabilized keys are on the Quiet Pro, however.
The key caps themselves seem to be about the same quality as the Das lasered/infilled ones. Nothing really stands out about them or detracts from them. The legends don't seem to be wearing / fading quite as quickly as my Filco ones have, but like all laser-infilled keys I've used, finger oils do still tend to make the white infill get dingy rather easily. I can see a difference already, as even after my first week of constant use, the number row looks whiter than the letter keys. Not really a slight against Matias, just an unfortunate limitation of the labeling technology. For this reason, I will always prefer PBT / dyesub keys if available. The Quiet Pro keys do seem like they might be slightly more resistant to getting shiny than the Das or Filco ones, but I think it's probably too early to tell on that yet. One other small issue, as others have mentioned, is that the LEDs under the Num Lock and Caps Lock keys do "bleed" out the bottom since the keys sit kind of high on the switches, more so on the Num Lock than the Caps Lock on my board. Not really a big deal for me, but it seems to bug some people.
Overall a great board at a fair price, especially considering all the R&D on the new switches and the fact it caters to a niche market. Thanks, Matias!