Painful, no. (not if you have the right setup, and don't try MAX as the first setting.)
A Cockring, or pad above the cock, as a receiving electrode (+)
A butt probe of some type as a sending electrode (-)
and turn the power up slowly. use a pulse or wave type setup. this will cause what feels like the PC muscle to work in rythym to the stim. after a short, or long, time (depending on your personal tolerance) you will begin to release pre-cum fluids. keep it going long enough and you will cum for real, and usually very hard.
mind you this can be made to be VERY intense (painful but no harm)
or an inexperienced hand can go too far and cause harm.
running electric current through the body is not ever a fantastic idea in the "safe" department
one other thing to keep in mind is that nerves will get used to sensation, and it will dull, and you will turn the power up, and it will dull, ....(repeat)
this is true both in one session and across many, give your body time to recouperate between sessions, and don't take individual sessions too far.
Last comment, different days = different bodies, you can alter your body chemistry by what you eat, drink, or smoke. how much salt/liquids/caffiene/prozac/alchohol/pot/etc you take in can alter your sensitivity. While I don't care about your personal habits and won't preach opinion at you for what to and not do before play. I will say this: Each day brings a new you. You will be more experienced at eStim, and you will possibly be more sensitive, do not believe that yesterdays settings will be dead on right for today, you may need more, but you may also need WAY less.
***GIRLS*** this works for you too, only the electrodes goes just above the clit, and just inside the pussy. It works your interior muscles around the g-spot as well as sending energy through the nerves behind the clit.