Flew this aircraft on AC861 (LHR-YHZ) and then AC615 (YHZ-YYZ) this past Friday in economy (seat 19C).

Seatback IFE:

• IFE is good in general (large screen, responsive touch) but many things just don't work right yet. Some of these non-working features probably depend on wifi (eg info on weather, connecting flights status); some are just straight-up defects (change the interface language to German or Japanese and see half the text still in English; "About this 737" information is missing; information on AC comfort fares and signature service is reversed).
• The moving map is perhaps more visually attractive then on the 777/787, but has less inflight nerdery available (eg units in nm/knots, airspeed, climb rate, etc).
• You have to watch something like 10 ads before a viewing a feature movie, which is abhorrent. You can skip them after the first 10 seconds, but after skipping about 7 or 8, I'd lost the desire to watch a film and went back to watching biathlon footage on my phone. Even worse is that you couldn't quit out of the movie interface and go do something else instead.
• On the other hand, it has Angry Birds, which I confessedly killed several hours playing.

Seating & such

• Legroom is quite good...given that I was sitting in 19C, an exit row.
• The ordinary preferred seats, although less roomy than the exit row, were also quite good in terms of legroom.
• No promises about legroom in the ordinary non-preferred Economy seats; I didn't get a chance to check them out.
• Width-wise, the seats are actually not uncomfortable; they are definitely better in this regard than the Y seats in the 777/787. That said, the stated 18" width seems to have come from shaving down the width of the armrests and possibly also the aisle.
• Seat cushions are a bit on the firm side, definitely getting uncomfortable after the 5th or 6th hour.
• Seatbelts are a good length; pax who just barely need a seatbelt extender on other AC acft won't need it on this one.
• Shoulder room still sucks, regardless whether you're in an aisle or a window seat.
• A good amount of at-seat power, even in Y -- a USB port at every seat, plus another USB & 120V outlet between each pair of seats. Also the USB ports seem to be at least 2.0A; they charged my phone much more quickly than the (presumably 1.0A) ports on other aircraft.

Washrooms

• The lavs are tiny, as promised. You can barely turn around; I've seen urinal stalls that are larger.
• Very gentle pressure on the tap is key if you wish to avoid taking an involuntary shower.
• There seemed to be constant lines for the lavs on these very full flights as well.
• All in all, definitely a plane to avoid much liquid intake on.
• The galley is also small and not very private for flt crew. I can't imagine the FAs are big fans of these at all.

Final verdict: would I fly on these again? I wouldn't go out of my way to book on them or to avoid them. I find them marginally more comfortable than most Y seats on the 777/787, though the miserable lavs are a definite trade-off. For domestic routes I wouldn't call them either better or worse than the current A320 and Embraer family equipment, just different. That's my assessment on the basis of things that are important to me, though; for different flyers, YMMV.