As a counter example I would say that I had a perfectly acceptable experience in Y on the 787. I've never flown business so perhaps it is bit of a perspective thing, but compared with other flights on other aircraft I found there to be a few cons but also a few pros to the experience. I flew EDI-DOH-CPT return on the aircraft.

The seats are tight, but I'm 6ft 2 so leg room is always an issue for me on any airline in Y. I found the narrowness of the seat not too much of a problem, but I'm of quite slim build.

On the other hand, compared with older aircraft, the cabin itself was a big improvement. The lighting effects and the lower cabin pressure really do contribute to you feeling more refreshed (or rather less fatigued) upon arrival. Somehow the cabin feels roomier and less claustrophobic than it would seem from the outside, in my opinion. I wouldn't recommend a window seat as the curvature of the fuselage and the IFE box does limit leg room quite severely.

I think like anything else it is a value judgement. As I said I've never flown in J, so the experience wasn't that different from other economy flights. For me it wasn't worth forking out an extra £800+ as I coped perfectly fine in Y and actually enjoyed some aspects of the cabin.