I've now had a few U.S. flights on these birds and they are horrible. It's not just that the seats are narrow and armrests flimsy, as noted above. The back feels much thinner so I can often feel the passenger behind me, which on a recent LAX flight woke me up several times. The corridor is uncomfortably narrow - walking to the toilets, one angles slightly sidewise but still brushes against the odd foot or arm. Service is slower and on my flights so far at least, upgrade likelihood seems to have decreased (this may or may not reflect the greater density).
It's not a setup for a premium airline and has already made me look at alternative carriers when I have a choice.