I have been playing around with it for a few weeks.

Dungeons and HM Primal:
Most bosses are affected by the slow. It will diminish on your pretty quickly when you pop featherfoot. It is on your GC, but it will not break your combo chains. Also it hits a little weaker then BB.

Coil:
Worthless as the bosses almost always never miss.

Its not horrible. But it looks much better in theory crafting than in practice.