I have found that the comfort level of any aircraft very much depends on the operator. The type of seat, seat pitch and seat width are the major factors plus noise levels depending on where you are seated in relation to the engines.
The B757 has always been one of my favourite aircraft and having travelled on 757's with BA, Thomson & First Choice, Monarch, XL, Eastern, Finnair, LTE, Delta and probably other airlines, I have found Jet2's B757's to be as comfortable as any and more comfortable than most, despite their age.