I had a pair made specifically for using in Neo Nyzul on my RNG mule. Since RNG evades like a rock, they work quite nice in there. I would venture to say I get at least 1-2 flee procs per floor which is very helpful. Often times I aggro enemies on purpose while waiting for the other team to kill a spec 1 just so I have flee on the next floor. I would also agree with the sentiment that the proc rate is roughly 10%. The main jobs I have on my mule are THF, RNG, COR and DNC and I never really use them on THF at all. IMHO you'd be better off with Fajin Boots for thf.
As a warning to anyone considering using the above tactic in NNI, don't do it. While my RNG performs exceptionally well in there, I only use it out of necessity and I have invested a boat load of gil into it just to stay on par with most cookie cutter DD's (Gandiva, Armageddon, Kraken Club, Aerion Boots and arrows/bullets that cost 400k & 1.8M per stack of pouches/quivers. It's really not worth it.