BHX - NYC
I've flown on both of these flights in the last 6 months, each one going out economy and coming back business.
The United flight has a lot of legroom in economy with an on demand screen in the seat back and quite a nice business seat. The business cabin was full and there were a handful of empty seats in economy.
AA are flying quite old planes on this route cabin interior wise, it's a long time since I've flown transatlantic without a seat back screen, these had a few screens in the middle of the cabin showing a programme of films. In business I think the seat was the one before the current one AA are replacing. The interior also looked and felt old.
Regardless of this > 50% of the extra capacity between NYC and BHX has been filled in six months.
One of the other results of this is it appears the United flights have reduced in price since AA introduced this competition. I remember a couple of years ago rejecting this United flight and going to Heathrow as an economy ticket was nearly £300 more plus I'd have had to stay an xtra day because of the flight times.
I think there is quite a lot of unserviced demand for flights to North America from BHX and like for like or even paying a bit more I'd always choose BHX over going from Heathrow, but there is a lot of choice a couple of hours drive down the M40 when you are getting old equipment/poor connections/no direct flights.