I am curious about this route, so I mapped it using MapBlast (www.mapblast.com) and it runs right by my house (I live outside of Soignies). Where the N57 from the N7 south to Soignies is nice (it is one of the best roads in the area, pavement-wise), is is straight most of the way. Hilly, yes, but nary a curve in sight. From Soignies to Manage isn't that exciting, either...mainly farm country and not a lot to see. Once you get south of Charleroi things start to pick up, scenery-wise, and once you get off the R3 it improves. The N5 south is OK but for all intenstive purposes I would recommend scooting to the east in Philippeville and head towards the Meuse River. The ride along it is pretty good and one of the best I have found in Belgium. It is a bit like going down the Neckar River from Heidelburg in Germany, only not quite as nice. Watch out for traffic, too!
This is a very good road, however it is badly marked on your map. However if you are in the area Frasnes to Lessines is the dogs danglies