My E-stim equipment is one of my favorite toys (ET312). Lots of possibilities. Great to play with others, and of course great to use on yourself, and let your fantasies flow freely.
Feeling is from very pleasurable tingling sensations, to more intense and powerful, at the edge of painful, to very painful. Nice is that during a session, you get more and more aroused, and you can handle more. Also, you can gradually build up some painful moments, if you feel like it.
Depending of the equipment, it can feel like an intense vibrator, a moving hand, or a penetration machine when used vaginal or anal. The contractions of your muscles in those cases make the electro dildo move a bit in and out, controlled by the device. This can be very pleasurable. Also ´whipping´ sensations are one of my favorites: very short repetitive shocks that slowly get stronger. Great for using while fantasizing some intense scenes.
There are audio files available on the internet that can be played through the estim equipment. They have whole sessions, with alternating between pleasurable and some more tough moments. All ´hands-free´ stimulation, which makes it great for exploring your more intense bdsm or medical fantasies.
It requires a bit of experience, and sometimes you make a mistake when moving electrodes around or changing audio files, and get some unexpected unpleasant zap.
Since two years I am also using the equipment during medical sessions when I am a doctor. All my patients enjoyed the experience.