The longest 757 segment I've flown is IAH-SEA, in both domestic first and coach. So I can't say much about the comfort level on a flight two or three hours longer than that. There are plenty of people on here who will tell you that it's awful; many of whom have probably never actually flown a trans-Atlantic 757 flight. The seats in coach really aren't much different from those on the 767s and 777s, and have the same pitch as the ones on the 777s. Unfortunately the 757s do not have personal entertainment systems in coach, only in BusinessFirst. The 767s and 777s have a television monitor at every seat in both classes.
I personally wouldn't mind trying the 757 for a trans-Atlantic flight, especially if I could get upgraded. But even if I couldn't, sitting on that plane for a flight of that length would probably be no more uncomfortable than a flight of equal length in coach on a 767 or 777. Sure there's less room to get up and walk around, but the 757s do have a nice long aisle, with a couple of areas at the front and back where there's plenty of room to stand up and not block the aisle or other seats.