I have 75 mbps FIOS and I think this would be bare minimum for decent quality streaming 4K video.
For what I consider good quality we will need Google Fiber.
Right now the cable companies are happily price gouging everyone for crappy 15-20 mbs connections, we'll see a lot of political battles before a significant part of the country will get internet speeds acceptable for streaming or downloading 4K movies.
And therein lies the huge hurdle. Is it worth it today? Absolutely not in the vast majority of cases. It is totally dependent upon what you want it for versus what you need it for. Need implies right now. Want implies whenever. If I don't really care about uncompressed 4K...or just 1080p upscaled to 4k...or limited content...or true 4k (4096p) versus almost 4k (3840p)...or wide technology integration like OLED, HDMI & 3D...or high prices versus cheapos...then anytime is okay. Now is as good time as any. Just grab your huckleberry and be happy. Because your impulse is based solely on what you want. But if what you want needs the stuff that is not available right now. You better wait. Because in a few years all will be readily available...with more brand options...and much cheaper prices because of inevitable brand & format wars.
Who wants to pay grotesquely overpriced cable/Sat providers more money anyway. Especially for inferior product compared to disks.