Kinda makes the feat not all that attractive, since not only the 15% chance is low by itself, the enemy can resist it too.
I found it to be pretty strong with a certain item. It let me handle large groups very differently. I could just run into 10 or more enemies, knowing that if I killed 2-3 of them, I probably wouldn't have to worry too much about the return fire for one turn. The AoE for the affect seems pretty large.
What difficulty? I know that death grin increases the chances, but on hard and dominating enemies are generally stronger too and probably will resist it more often.
I was playing on dominating. I basically took the feat for fighting against groups of 8+ enemies, where it was likely to have an effect, especially since I was playing with 3 CON and light armor. So when I say it was strong, I mean it was strong fighting against large groups, since I wasn't looking for it to be useful in other situations. After all, fights with fewer enemies were pretty easy.
Anyways, the feat was disappointing before I got Death's Grin. Or maybe my luck was poor. Either way, I stopped leveling Intimidation since I thought it wouldn't provide further benefit, so my effective skill (with Death's Grin) was only 105. I don't know if dominating raises enemy resolve. Maybe it does, but I was playing a sword/knife build, so it was pretty easy to kill 4-5 enemies each turn. And that meant 4-5 chances to apply fear so the feat felt pretty reliable against large groups.