Just did YVR-LHR-YYZ this week. A/c, Service and food were good. The only glitch was an equipment change on the return, so they tried to bump me back into Y. Because of a very helpful concierge @ LHR I was able to rebook an I seat on an earlier flight, so it worked out, but only by fluke.

^ to AC, and especially the LHR Concierge.

Can't speak about US carriers because I haven't done it.