I've ridden various chunks of the route in the last year or so. The path parallel to the A9 through the Drumochter Pass is in a very poor state, but there's no way around it.The South section was fine (except a pointless loop up over the hills between Maybole and Ayr instead of going straight down the flat B road ), though I lost the route in Glasgow and just headed for the Station on road.
I've only done bits of the northern section. I wasn't keen on the bit next to Dromochter Pass when I rode it as part of LonJOG in 2012, but it may have changed. That it was chucking it down as we were riding over the top probably didn't help my memories of it either.