I dislike the stun baton immensely. I dislike its look, I dislike its sound, I dislike its swing animation, I dislike its intermittent recharging and I hate that it has it's own tree pretty much dedicated to one weapon, especially when it's just so awful. These are, of course, subjective opinions.
However it is definitely not OP, except in one very specific situation.. and that's horde night. If you have the perks, the candies and the armor, then a top tier stun baton is literally unbeatable, even up at Insane difficulty. The simple fact is you can't nerf something just for the horde night, and other than it's zapping ability, it's just a @%$# club.