Ed,
Durability would not be a strong point (same goes for the tails on any Pheasant Tail nymph I've ever tied, although they continue to catch fish). I tied these true to the patterns I saw more for fun while playing around with flies that are new to me. Subbing knotted rubber, antron, antron/pheasant, etc. would definitely produce longer-lasting legs.
http://www.caithnessqualityflies.co.uk/
Regards,
Scott