I've never experienced this and there seems to be a lot of conflicting info online about safety, intensity, possible burns etc etc. I did have an online conversation one time with a west coast Domme about an extended captivity style session featuring CNC. She is something of an electro specialist and mentioned she often would use her cattle prod a couple of times in the early part of this type of session to firmly establish the reality of the sub's situation and to demonstrate her control. Unfortunately my travel plans fell through that time and the session (one of my bucket list type sessions) never happened.

I recently came across one being used in a video and wondered about the reality.

So, has anyone here who has expertise or who has had first hand experience with these devices? To be clear I'm talking about actual cattle prods and not BDSM toys like Tazapper or the Stinger.

No, but I see a domme once in a while who says she uses these on serious masochists . . . I think she has photos of her prod or prods in some of her bags of goodies, which she has shown on her twitter feed!

Belle d'Jour , the ancient domme house uses one. It packs a wallop, should be OK if your domme sticks to major muscles, and threatens to use it on you balls if you don't climax in time.

where is belle du jour and who are some of the dommes at the place?

While very scary when applied to the balls, it is actually more painful when applied to muscles.
Just beware of secondary risks, you will involuntarily jerk strongly. If the balls are tied to some heavy object, you may say good-bye to them.

The Belle is in NYC, the mistress? I forgot because this happened 27 years ago

Alas, the legendary Mistress Belle de Jour passed away a few years ago. From what I read she was in her mid-90s and still used to go to a BDSM event or two. The last of her dungeons must have around 27 or so years ago, the late '90s.