Okay, you asked for it...
I bought the DVR in March of 2000 and have used it quite a bit. I won't say it's perfect, but the only problem I've had with it is that audio can get out of synch sometimes. Other than that it's been my most consistently reliable realtime encoder.
I've used it in WinNT4 and now in Win2000 for the last year (been using the latest drivers since they came out a long time ago). I've made VCD's and DVD's with its captures. I originally bought it to capture DV camcorder footage to MPG2 so that I could make DVD's (home video of my baby daughter). I didn't want to go the firewire/AVI capture and then encode in software route. I was willing to sacrifice some video quality to have my MPG's immediately.
From my point of view the quality has been pretty good. It really depends on the source. It does not do very well at low bitrates. It encodes well lit DV footage pretty flawlessly. I also use it as a digital VCR (or PVR, like a Tivo) by feeding TV to it through an S-VHS VCR and then using Girder to start and stop the captures on a timer. It works pretty well.
I only have a couple of complaints about it. One, you can only adjust the encoding bitrates in 500kbs increments, which is rather limiting. And two, the aforementioned audio synch problems. The audio synch problems are the most aggravating, but frankly every realtime MPG encoder I've seen has had some problem that's almost as debilitating. I can fix the audio problems in Womble if I really want to keep the file. The audio synch problems are directly tied to video source quality and so only appear when I record from TV, and then happen randomly depending on the show and the strength of that TV channel. I now feed digital cable to the SVHS VCR before going to the DVR, but it didn't seem to help that much (some shows now do better, others do worse). Time also affects it. If it's a half hour show I may not see any drift. An hour long show will get audio problems typically more often than not by the show reaches the end. I think it's the sudden changes in the commercial breaks that affects it the most. It's very sensitive.
On the other hand I also have a Radeon AIW that I'm using in W2K with an Athlon 1900XP. It's pretty reliable too, but it pads the MPG files in a weird way to make up for hidden frame drops. I never get audio drift with the AIW, but I can never edit the files in Womble either without the audio REALLY screwing up (like being delayed by many minutes or worse, but only once I bring the file into Womble). So the AIW files while good to watch back to TV are not generally good enough to keep (I also have a Hollywood Plus with a remote control in my family room I use to watch all these shows, over the LAN). There's supposed to be a tool in the new versions of Womble that can fix this AIW problem, but I've never been able to get it to work for me. Maybe our local diety, Rich, can explain that one for me
The AIW does produce much smaller files due to its new aggressive VBR and is much easier to use overall (and sometimes seems to produce smoother, less grainy encodes), but until I can figure out how to fix the software file padding thing, it's the DVR for my important stuff.
The Sigma files are the best for using to make DVD's, in my opinion, because the encoding is so consistent. It never drops frames and it never messes up the video stream. The audio stream is another matter, but it's far easier for me to fix that than a messed up video stream.
I've heard lots of people complain about the DVR, mostly because of it's extreme Macrovision detection. But since I don't record from VHS tapes, I've never had that problem. Oddly enough when I played a Macrovision protected DVD from my Hollywood Plus machine back to the DVR machine's inputs, it recorded fine. Guess Sigma products ignore their own Macrovision! The other complaints I've heard are that it's hard to install. I never had a problem, but then I always made sure to use 440BX motherboards as well. Supposedly they've resolved that somewhat.
I was pretty psyched to hear about the forthcoming Radeon AIW 9700 Pro and its 'hardware' MPG encoder. But then I found out that it's not a full hardware codec, it just offloads about 20% to the video card leaving the CPU and the software to do the rest. My favorite thing about the DVR is that it's all done in hardware. For me that makes it all worth it. Here's some other reviews of it, one that's pretty kind (and also likes the AIW a lot) and another that's not so kind:
http://tangentsoft.net/video/mpeg/reviews/
http://www.tecoltd.com/enctest/enctest.htm