kennyp - the Nail Trail is pretty boggy at the moment up the top section, however you can extend it by riding parallel to the road heading W from the startpoint as there's some singletrack all the way through the forest, then taking a right and following another tree line. This option is drier as it is used less, but has a few fences that breaks it up unfortunately. It's not as flowy but makes for an alternative.
I like that way - the initial singletrack beside the road is flowing then there are some trickier, rooty bits later on. Closer to Balerno there are some more flowing & faster bits again. I've never actually met anyone else riding in there (just the odd dog-walker etc) except when I was taking mtbscotland rides through there - and even then perhaps only 2 or 3 times.
Might go up there tomorrow morning.