We got ours about 3 months ago. We got a 65" Samsung. I'm not usually one that takes the saying "night and day difference" too seriously, but this is. We had a 50" Pioneer Plasma before, and the new Samsung is one great upgrade. For the first couple of weeks, we kept getting freaked out because it looked like the people were right in the living room with us. It's still like that sometimes. The picture is crazy good.
We did get a 4K movie and the detail is...what did SeilerBird say...staggering. We watched Gravity. The movie itself was pretty slow...good, but kinda slow. But the detail of the 4K was awesome. The big drawback to these...I hesitate to call them a "TV"...is the sound. Because they are so slim, you just can't get any kind of sound out of them with the built in speakers. It's like going to a 3D High Def movie theater but listening to the sound on a walkie talkie. So you have to get a sound system for it. We got a Sony sound bar with a subwoofer, and it really brings things to life. I'm sure a full features surround sound system would really expand it to it's full potential, and I'll probably do that someday, but for now, it is good enough for us.
The other reason we finally went for broke was we were tired of external devices for getting to Netflix (and other stuff). With the Smart TV, all that is built in, and it works great. Netflix streams smoothly, and is much easier to get into since it's built into the TV. Other apps are included and are fun to play with, but Netflix gets fairly abused.
So if you've been thinking about it, but are not sure, go for it. You won't regret it.