I cycled this part of of the National Cycle Network last week (Starting on NCN 47 just south of Blackwood to Quakers Yard & then north on NCN 8). I started out in the cold rain but then the sun came out, and then it got foggy and damp, but then it was sunny and humid as I finished... "Welcome to the Valleys", as I've been told.
This is a great ride with a wide assortment of interesting things to see. The viaduct at Maesycwmmer is perhaps the most notable. (And yup... the bike path runs right across the top!) I'm going on a longer trip tomorrow; the tail leg of which will be this path. I'll be sure to get some photos to post and update.
Note: Merthyr Tydfil: Birthplace of the Industrial Revolution. Also: The Wheel o Drams is an impressive sculpture on the eastern side of the Maesycwmmer Viaduct.