I always advise to find someone who has a unit, preferably a good one, and ask to try it, so you get to feel what it's like before you spend money. You can use disposable medical gel pads as the electrodes. They work quite well, they're cheap, and you won't have to use someone else's.
Most people recommend starting with something very cheap, but that doesn't really give you much of an idea of what a good sexual power box feels like. I compare them as like a golf cart vs. a sports car. Do you really get much of an idea of what a sports car is like when you're in a golf cart? Most people, after trying a cheap unit, only go out and buy a much better one, and so the original investment is wasted.
A good sexual power box can produce very erotic feelings, different vibrations, rhythmic contractions that run from one electrode to another. Cheap medical units just go beep beep or buzzzz. That's enough for make some people cum. For most, tho, it takes something more sophisticated -- and then who wants to cum quickly when it feels so good?
After the first time I fell asleep with my ET-312 running, I posted a question about this, and discovered that many people, including women, often plug themselves in with the power on low when they go to sleep. It's very soothing to fall asleep with your genitals gently pulsing to some nice rhythm.
If you come to New York City, I would be happy to demonstrate a few for you.
The ErosTek ET-312 is universally thought to be the best in the world ($600 new) and it is very flexible in what it can do since it is totally digital. People who go to sleep with it on usually do so using the Waves mode. After some years I bought their ET-232 ($300) just to see what it was like. I was surprised that it's rhythms were different from the 312 -- even when they had the same label. I discovered that I really like the rhythm at 3 o'clock which is labeled Thrust. It has just the right combination of vibration and traveling contractions !!! I think a woman would love it with one trode on her clit, and the other across the bottom of her labia.