It's painful, but the more you do it the less it will hurt and the less it will react. Also, the longer you are on HRT (and blockers) the thinner your hair will be, too, and so will be your skin and the depth that the hair is in your skin.
Being post-op I find it doesn't hurt as bad as before when I tried it earlier on in transition. I just decided to try it again after the epilator was sitting there for a couple of years.
I've tried waxing (messy) and sugar waxing (not as painful and less messy than waxing). I like sugar waxing, cause it's all at once... but it takes practice to do it well... and not all area's are as easy to get to if you're doing it yourself vs if someone else did it for you (like a professional).
I tried my armpits and it hurt and I won't do it again with the epillator. Plus, i ended up cutting myself with the epillator when I tried there. Be careful.
I don't recommend it for the face... I tried that earlier in transition -- bad, painful, not good! It's cause the hairs were thicker, and deeper in the skin then. I've tried it recently just to see, but I just have peach fuzz. I still have the odd darkish hair (haven't been able to afford laser in a long time), but the hairs are very fine, and no one ever notices them... people think I'm insane for noticing them, but I' also looking at them through a 10x mirror. o_0 I thought I'd give it a shot, and it didn't hurt much (much less than my legs).... but that's peach fuzz. Lesson in all of this" The finer and thinner the hair and more to the surface it is, the less painful it is.
The hair on my legs and arms is pretty thin as well. Upper leg is more sensitive, although. I couldn't do my upper leg earlier in my transition at all! Now, it's still sensitive but easier 'cause the hair is much finer. Arms, no probs -- but hair is very fine there anyway... even when grown out, not noticeable. I know other women with much hairier arms and legs then I have. No hair growing else were, except the normal peach fuzz body hair -- I don't even notice much with the epilator if I put it anywhere else, but no point.
Armpits, finer than before, of course, but still, a very sensitive area! Eeeks! Of course, with eye brows, only pluck those or get them waxed by a professional.