I have a pair of 2013 K2 UFO's. I had such a hard time finding a fantastic boot, that I may pick up another pair if they go on sale later this year. They are the most fantastic boot ever. I swear I could ride all day and then hike 20km back to the parking lot without any discomfort.
I haven't tried the ufo but i've come this close to order a pair and within a few blinks of my eyes they went away...
so then I decide to get the dc status boa.
the good side of K2 is that they are extremely comfortable, although its bulky build can be a bitch with some toe straps.
the DC has a better shape, kinda closer to my slx, but way more comfortable and best heel hold ever; that's what it matters to me after going through 5 pairs of different boots.
anyway, UFO and Status boa are the best stiff boots in the market in my opinion