I've flown in the rear UD cabin on the A380 in WT 3 times this year and find it very comfortable (at least in comparison with WT seats on the rest of the fleet).
Of course, it's never going to compare with the CW experience (and yes, it hurts to be 6 hours into an 11 hour night flight without a flat bed seat!), but for economy travel it's pretty good. Personal space is better on the Upper deck - the side pairs of seats are nice and the side bins very useful.