My first experience was yesterday on AMS-HEL. At check-in, EC was completely empty, just a few seats taken. By the time that boarding was completed, every single seat was taken, including all EC seats. I don't have any evidence, but must assume that most of those seats were assigned to the last people to check in, as many of them did not seem to be frequent flyers.
I'll be watching this closely. As a gold elite, I see no reason why KLM can't at least offer these seats for free at check-in. This seems a reasonable compromise as they'd otherwise just be given away to whoever needs a seat at the last minute. If I'm going to regularly be relegated to the back of the plane, I'll drop KLM like a bad habit.