I'll chip in here...
I bought the 4k curved 55" 8500 Samsung last April, quite frankly I've been nothing but impressed with it.
for "regular" HD content, the picture is crisp, but for either 4k content or viewing my photo's at full size, it is simply incredible.
When I bought it 10 months ago there were cries of "no content available", but even back then there was... and now, even more so.
There are 4k films on Netflix, and youtube has hundreds (if not thousands) of 4k clips - and of course with things like the go-pro, some smart phones, and the latest DSLR's also filming in 4k, the content is only going to get more varied - And yes, I have watched quite a lot of it now, and it's very impressive!
Also, HDMI2 standard is set and released, so data transfer isn't an issue either (another 'against' argument I was given).
As for 3D, well, I didn't get a choice, it came with the TV, but the 4K means that the 3D image is "Full HD" 3D instead of 720 (which you get on a 'normal' HDTV)...
I can't say I'm a serial user of it though... infact I've only watched one film on it - Planes, Fire & Rescue. But that said, the 3D effect was bloody good!! Only downside was indeed the glasses - but given the infrequency of me using them, it's not a killer for me.
Am I glad I bought 4k? Yes definitely.
Should I have waited? possibly, but only because I could by my same TV for £1000 less than I paid 10 months ago! LOL, but that's the price I pay for having stuff on release day
I'll probably upgrade when OLED 4K comes out, but thats at least a few years away yet (in sensible prices anyway).