Over the last few weeks Sears has run some excellent deals on Seiki 4k UHD TVs. Some of the prices I have seen are as low as $589 for a 50" and $599 for a 55". They are almost always available for pickup at a local store (I have seen them at McKinley, Galleria and the mostly vacant Summit Park Mall) before shying away from being an early adopter of 4k, one must realize that these prices are scraping the bottom of pricing for 1080p 120hz sets as it is. You get 4k as an added bonus. While the sets only support 4k at 30hz, unless you are using it for gaming it won't matter, movies are filmed at 24hz. Additionally there never has been confirmation by Seiki that these sets won't support HDMI 2.0 via a firmware update, particularly the 55" set that uses a V by One panel that technically can support 4k at possibly as high as 120hz.
I took the opportunity to buy the 55" when it was selling for $599 and the biggest drawbacks are black levels and over accentuated color. As for the black levels they're not bad, but not the best as this is an edge lit LED without local dimming and I am a spoiled Pioneer Kuro owner. Out of the box the color is overdriven, however once you set it back to 37 the TV looks pretty good.
The smaller pixel size does help with good Blu-Ray and HD DVD source material and it is noticeable despite what some will say, I can notice it at my 8 foot viewing distance quite easily.
Despite limited 4k material more will become available. Seiki will be sending registered owners a U-Vision cable which will upscale existing content to 4k.
If you are looking for a cheap 1080p set in these sizes I would watch Sears pricing very closely (it fluctuates online almost daily), you could walk away with a 4k set for roughly the same price. Walking away with 4k for the same price as a 1080p is a deal that is hard to pass up.