This was a route a friend and I stumblled across whilst in Belgium/France for the weekend.
The route starts with fairly long straight sections through the countryside with nice open fast corners, but as you approach Saint-Omer you go into a forest and roads become a bit more twisty. So the route has a bit of everything. Personally I liked the strech from Steenvorde to Saint-Omer most.
We did it on a sunny Sunday in May, and there were so many bikers out along this route, and virtually no cars. Its as if the entire population of France gets on a bike on a sunday! The route was not busy at all either, traffic was virtually non-existent.
When you arrive at the square in Saint-Omer indicated on the route, there are usually a few bikers hanging out and cruising around.
To be honest, this route was great, but I reckon a lot of the rural roads in this part of France are equally as good, so just go out and find them!