Another Darbee data point...
I received my Darbee today. I installed it, which took about 3 minutes.

I watched a bunch of the Olympics and I thought the effect was subtle... until I turned it off. Then the picture seemed really soft. Lesson: If you're going to evaluate the improvement with this device, do it on content with which you're already familiar.
So, I put in my "most familiar" piece of content:
The Eagles,
Farewell One on HD DVD. I keep my HD DVD player in my system *just* so I can watch this disc. It's my single favorite concert disc. I owned on it DVD and when it came out on HD DVD, I bought a player just to be able to watch it in Hi-Def and listen to the DTS-MA track. I've watched it literally dozens of times, having used it as my primary concert video demo disc. I know it inside and out, so it was very easy to see the difference with the Darbee.
Adding the Darbee improved the video
significantly! The most noticeable improvements were sharpness in the details of facial features, and the textures of the clothes the guys were wearing, and the grains, textures and wear-marks on the guitars and violins. Also, when the camera does long pans of the audience, the detail in those pans was much improved. In addition, I thought the shadow detail was hugely improved. Usually to get better shadow detail, you need to raise the black level a little. No need for that now. Shadow detail is much better AND blacks are still deep black.

My biggest concern before seeing the device in use was that it would add some EE ringing or other artifacts. I didn't see *any* of that, (unless I turned it all the way up.) Running it at 65 or so just added detail, sharpness and an increase in perceived detail.
Overall, on first blush, I'm pretty impressed.
One editorial comment though... for $249 they could have at least thrown in an extra HDMI cable. Everyone who gets this thing will need an additional HDMI cable. I had an extra one laying around, but many people won't. Why not just include one? Hell, they can buy them in quantity from PE for peanuts: here's one for $3.50 if you buy more than 4:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=181-772 I don't know, it just seems cheap to not include one.
Other than that, I'm pretty happy.
Craig