I've been skilling up Staff for DRG in Kuftal with my Kinkobo. Just got the +1 and have been field testing it. From 506 hits, I've gotten 26 stun procs so far, which is just over 5% proc rate. It CAN proc stun on jumps. Seems to be most effective with a Soboro, as the stun can last upwards of 7-9 seconds.
Did some further testing solely on Lightningday (to see if the corresponding day improved the proc rate). I got 14 stuns out of 262 swings, which is still only 5.3%.
Other possibly amusing uses would be a MNK with a Mercurial Pole using Hundred Fists. Should be attacking fast enough to proc stun pretty often. :D
Used this in a merit party last night attached to my Soboro Sukehiro. Even with all the hits of that sword, the stun didn't proc very frequently - maybe 5-10% of the time. However, it lasted for several seconds (3-5 usually).
The proc rate isn't too good (5% seems about right), but the stun lasts quite a while. This can be a nice side effect while soloing, especially on tougher mobs, but I wouldn't rely on it.
I don't see it being very useful in an EXP party though, especially since most people use single hit weapons (except maybe for Soboro). And even if all DDs use Soboro and equip that, it would still only proc once every fourth/fifth attack round, which isn't too good especially for long delay weapons. So for that it's probably better to just go with a Pole Grip.
Other than that, it can be helpful, I used it on PLD with a Mercurial Sword (1-4 hit weapon) and it helped me get some cures in or recast shadows, which is nice. But as I said, you shouldn't rely on it. Just have to watch out for the stun and make the best of it.
