OK, probably not too polite a comment. But I've read rave reviews on a number of US boards about after-market seats, be they Corbin, Sargeant, Russell, whatever. And then you realise that the guys doing the raving are, like, 13st or even more and you have to wonder.
But, then, I actually like stock BMW seats. If I can do a 14 hour ride without feeling it then I can't see the point in shelling out large sums of cash for a different seat.