Having previously said my other half found them uncomfortable, now having used them myself, I think they work quite well.
Lets be honest, they're not comfortable, but the height and the pitch make it bearable. We recently went from Bedford to Gatwick and only around Elephant and Castle did I start to get a numb bum.
On the way back, no worries though, which was odd.
I also suffer from shoulder problems as well as back problems, the 377's (2+2) coaches are lovely to sit on, but are murder for my shoulders as the armrests appear to be around my nipples.
319's were soft but far too low so knees clashes and coccyx's painful.
I think 387's are the best we're gonna get tbh. If they had a slightly concave seat, I think they would be perfect