I have only flown United twice, and that was a round trip JFK-LHR, and I must say that I was very impressed. The 767-300 was roomy, the inflight entertainment was very good and the service was nothing short of excellent. The cabin was clean, the flights were on time and the ground staff at both Heathrow and JFK were very friendly and knowledgeable. Having flown this route with BA, VS, and AA and JFK - AMS with KLM, I would not hesitate to use UA for this flight again. Here are my reasons:
AA - the 767's were ok, but the A300 I felt were rather tight and small, service was good, but the lines at JFK were monsterous. Service was good.. United was better.
VS - I feel that no airline goes out of their way to make you as comfortable as Virgin, however, if you are on one of their 744's for more than 6 hours, you want to be very short and very thin, especially if the flight is fully booked. (and when is it not)
BA - Another good airline, but the same boat as VS, if you are lucky enough to cross on the 777 you should be ok, but the 747's leave a lot to be desired.
I have never flew Air India on this route, and I am not sure that I want to.. - any comments?