OK - I've been using this screen as a monitor now for about a month. I would say that it has met about 90% of my expectations.
My setup:
ASRock Extreme6 motherboard
4960X CPU @4.6GHz (liquid cooled)
32GB 2400MHz RAM
ASUS GTX Titan
Windows 8.1 Enterprise
I replaced 3x LG 27" flatscreens with this 40" beast and sit ~2.5' from the surface (if watching a video I tend to push back another foot of so and put my feet up on an improvised ottoman under the desk). I don't play a lot of games these days, but Skyrim looks awesome in 4K - with the benefit of not having the "blank space" in the form of the bezels between the screens getting in the way of my view. The Titan now has to render 33% more pixels (6mil -> 8mil), but it handles it.
The only caveat I have is that some thin fonts have a color shift from black against a white background which is slightly noticeable from my standard viewing distance and easily seen if you get within 12". Against say a pale blue background the same font looks fine.
Solidworks displays look "solid" on the this screen albeit some small, thin fonts have the same issues as above.
The screen handles 2160p input from HDMI @60Hz with no hiccups.
I was able to compare this to a Seiki 39" 4K TV - the Seiki sucked - and I mean badly. The fonts on the desktop were blurred to the point of being illegible. Trying to view simple webpages or read Outlook email was painful until you bumped the magnification up to 200%. Avoid that abomination like the proverbial plague.
I picked the LG up on a Black Friday sale at Costco for $589 CDN (that's $71.43 U$D I think =P) and can still return it within the next 50+ days if something better lands, but if not I believe I'll remain happy with my decision to get away from the 3 screens and consolidate to this one.
It is also connected for TV viewing (I use over-the-air broadcast) and all the HD channels look fine.
Overall a solid 9/10 for me.