I'm a vet(no longer in active practice), I have one, and I use it on occasion - probably not more than a couple of times a years. Our cattle are reasonably gently, but when trying to get less-than-cooperative cow or heifer to take a calf, or to treat some malady that I can't do(safely) by just sticking 'em in the chute, it's indispensible. I'll never try to milk out a bad quarter or graft a calf on again, without using it - takes too long for my hands/arms to heal after the cow breaks 'em kicking or stomping on 'em.
Cows that I've used the immobilizer on - and sometimes they've gotten it a couple of times a day for several days in a row - are not any crazier afterwards, or any more or less cooperative, when it comes to handling them.
I've hooked myself up to it - it's not painful - though, admittedly, I didn't put the electrodes on my own lip and anus. My wife, also a veterinarian, and a certified veterinary acupuncturist, has 'needled' me on occasion, and has used her electrostimulation unit on the needles inserted into some acupuncture points, and I'd say the sensation is similar.