I think KLM has a (not so hidden) secret in its long haul fleet. For the cost (generally cheap even if you pay full price to upgrade) - which is free to highest level KL/AF/DL members - Economy Comfort is proportionally MUCH better thant KL's Premium Comfort.

KL's Premium Comfort is one of those (sadly many) Premium Economy products out there that suffers from rather unimpressive usable legroom (regular rows, not front/bulkhead rows). I am just over 6ft, so have legs to go with, but this is also a fairly normal size within the standard ranges. Yet the boxed-in space underneath the seat in front, plus the hated and dreaded fold-down foot rest (surely this is only for shorter than average pax) providing a hard metal obstacle on the top, make this a very uncomfortable space for my legs/feet.

The usable legroom, in which to twist and turn, plus the added recline in Economy Comfort - both better than regular Economy - plus the not impossible chance of an empty seat next to you unless the flight is full
- make this a much better option, in terms of value for money. Which for many means no money at all as it is a standard benefit for highest level Flying Blue (and DL) members.

Save your guilders for better uses!