There are many threads ranking various products...and many opinions as well.
for just the SEATS, United is narrower and harder than some of the competition.
The two best I've flown that are truly 180-flat are Air Canada and New Zealand. I think South African was also 180-flat, but all I know is that I slept for 11-12 hours straight and so I guess I must have liked it
United's implementation of 180-flat is WAY ahead of ANY angled flat I've tried (Thai, ANA, Singapore (old), Cathay Pacific (old) come to mind...can't remember offhand if I've been on any others).
The point about direction and "tilt" of the aircraft is correct. I did notice a difference when I took the rear facing seats and found that it was easier to sleep. Never noticed that on any other carrier's lie-flat seat, but then again it may only be because it was pointed out to me recently and I've only flown UA since.