Select Seating is KLM's equivalent of UA's E+ without the legroom, that is, seats at the front of the economy cabin when flying long-haul on an economy fare. I seem to remember KLM implied there was more legroom, but I never noticed anything more than usual.
The only benefit is faster deplaning, and the company of more experienced flyers over that of tour groups. However, you are closer to the babies too

You can request it in advance when making your booking for long-haul flights; my success rate when flying on NW metal has been limited. On KL I usually get a good seat.

On intra-Europe flights, there is usually no advance seat assignment; however as a FD Elite you have a fair chance of getting a business class seat (2+3 across instead of 3+3, not bad at all) on 737's, as the size of the C cabin is determined by a moveable curtain. There seems to be more of those seats allocated to Y on older 737-300's than on the new 737-800's.

[This message has been edited by monahos (edited 01-05-2002).]