Wow, I have been avoiding the incomplete path and taking the other bad choice of waiting for the southbound slip road to clear before using it to join the roundabout. However tonight heading home from Rosyth direction I thought I would find out exactly what would be on offer if I followed the signed route for cyclists. Coming from Rosyth I didn't see a single cyclists dismount sign but that was pretty much the only positive.
The route starts at a the foot of Castlandhill road with a Toucan crossing (the other positive). The surface almost immediately becomes compacted hardcore with raised manholes surrounded with sand. Too keep you awake you then have to pass through a pair of shoulder width portals either side of the motorway slip road (Even the SNCA wouldn't consider that on their courses)before it spits you out in front of any busses who are using the park and ride. It carefully routes you behind the Park and ride building so that you can't see any buses which are coming before releasing you straight onto the road without warning.
At this point I didn't see any signs so had to guess at turning right and heading to the entrance where I could pick up the compacted path again. Now heading towards Inverkeithing I crossed the zebra crossing and finally turned back towards where I wanted to be. As DaveC has said the path then does a series of wiggles round some barriers and other obstacles before rejoining the old path.
Other than the road crossings and bus forecourt it would actually make quite a nice (if short) route at a trail centre. As a commuter route though it's more like sticking the traffic from the M90 down a farm track.
I'll be sticking to using the roundabout and if Transport Scotland don't want cyclists in the way of their deity then they'll need to think again.