My experience, using NW miles for a KLM operated flight has been pretty good, in fact I am traveling IAD-AMS on KL in September and booked the flight using NW miles and on the return snagged a DL business elite seat AMS-ATL. I'm looking forward to the new KL A330-200 service from Dulles to Amsterdam.
FWIW, I booked this trip on the NW web site this April.