I disagree , If I had the choice of NAX/JBU or one of the legacy carriers it would be the latter every time even if it was on one of the aging 757's.
Yes , the lead in prices may be tempting but with the various add on's are you really getting a bargain? And also the pax going transatlantic is rarely point to point so the Alliance carriers have the advantage of hubbing whereas the others will have to organise a whole host of ticket interlining.
NAX maybe having some success from their current PTP routes but we are talking London and to much popular places like NYC/LAX/FLA , it will be much different from the regions.
I hope UA can hang in there but if not and some carrier like NAX is the only alternative then it's BHX via CDG or AMS westbound for me.