United (& previously Continental) 75's have been doing the run from Edinburgh to Newark pretty much since Edinburgh got a regular transatlantic service. I've lost count of the number of flights I've taken on them across the Pond.
As has been mentioned on many other threads, the 757 was built with power to spare and is very robust, the only real reason that there hasn't been a "new" 757 is that the design was very good to begin with and many have stated that the only possible replacement for a 757 is a new one, giving rise to speculation over when Boeing might reconsider opening the production line once again.
The only reservation I would have about wholeheartedly recommending the flight to you is that United use their planes very intensively, so, the interior of the plane may not be the freshest, but it is very well maintained and safe. The service on United, isn't quite what it used to be on Continental, but it is pretty average for a US carrier. i.e. it's OK, but, just don't expect a lot for free or to be looked after like you're in first class on a gulf carrier.......