After starting with some Zeus box thing that I wound up tossing in the bin, I've been stimming with an Erostek ET-312 for over 10 years and I can say that it's nearly impossible to describe exactly what erotic electro-stimulation feels like.
Depending entirely on your own personal nerve wiring, the placement of the electrodes, and the settings you use, it can feel like any of the following on your sensitive bits...
A feather-like sensation.
A lightly pleasant sensation.
A definitely pleasurable sensation.
A really pleasurable sensation.
An "OMG don't let this stop" pleasurable sensation - like microscopic vibrators moving around under your skin and through deep tissues.
A little too much pleasurable sensation.
A this doesn't feel all that good anymore sensation - now entering BDSM territory.
A please-make-it-stop sensation - serious SM and beyond.
And that's just hands-free. It can also amplify external stimulation and make masturbation or sex with a partner feel ridiculously incredible. (Disclaimer - e-stim sex with a partner can be hazardous if current paths aren't properly minded.)
It takes a lot of experimentation to find what works for you, and there will still be more fantastic combinations of electrode placements and settings that you haven't tried yet.
Start out with inferior equipment and you're likely to be discouraged and walk away. Work with quality equipment and over time, you're not likely to be disappointed.