I went into a local electronics shop over the weekend to take a look at the 4k TVs. Some of them were setup with 4k content (wow!) and one had SD content being displayed, the image degradation didn't appear any worse than the 1080p sets, so I'd quite like to see a 1080p image to see decide whether they're worth an early purchase.
I'm currently swaying towards a decent 1080p set with a good response rate instead.
This is my belief - the 4k screens will only get better, but the advantage you get out of them is limited for the livingroom - the content just isn't there. Netflix has some 4k support though, but I skeptically just look at the datarates, and as long as a regular BluRay has 8x the datarate of streaming video at 1080p already (though Netflix can look really good at times, just as often it gets worse because their servers can't handle the load at primetime always), I'm not keeping my hopes up for the quality of streams just yet (not to mention audio sacrifices). I prioritised response times and went for non-3D, because I noticed that most TVs that support 3D have worse displays (worse black levels, for instance), at least until you pay almost 3x the non-3D panel price, and while I really like passive 3D a lot, the nature of how that works has in all the panels I've seen always decreased the quality of the 2D image. In that sense, I would have better hope for a 1920x2160 panel with passive 3D.
On the PC, right now I think having 3 good 1080p panels next to each other is better for gaming than a single 4k screen for sure, but that partly depends on the type of game you play - I'm sure some strategy games are much better in 4k on a single screen.

Few games have the assets to scale nicely though. For business use I would definitely appreciate 4k for clearer text. This would currently be my first and foremost reason to want to upgrade - it's why I waited for iPad's to support something much better than the 1024 they started with, and the 2kx1.5k pixels that came with the iPad 3 definitely did not disappoint, and were the best visual upgrade I experienced.