Bryony,
Aha!! A fellow masochist-to-be! I've used this interesting little torture instrument for a couple years now, off and on. enough that I'm on the second one. A similar model by the same manufacturer, rechargeable. Works quite well, as long as you take your time.
A few points of advisement: If your hair is more than 1/8 inch long, use an electric clippers to shorten them first. If they're too long, you'll end up breaking a lot of them off at about skin level rather than pulling them out. Work against the grain, slowly, with your skin semi-taut. If the skin is held too tautly, again you'll end up breaking a lot of them off. Incidentally, this is what causes problems when trying to epilate your groin area, If you don't manage to hold the skin fairly tautly, you can suck that scrotum skin right into the tweezers, resulting in a mass of small (and very painful) cuts in the skin. However, you'll only do that once or twice before you figure that out!!
Knees are a strange area. No matter what process you use, you're going to break a lot of hairs. There must be something about the skin in that area that's different, since it's meant to handle a bit more wear and tear, and the hair as well. While the hair on the rest of my legs is 90% gone when I grow it a little to check, I've probably only lost 20% on my knees. Odd.
I'll keep my ears open, since if you use it on your face, I'll probably hear you on this side of the "pond". Tried it (just Had to), and with the first few hairs I knew it was not going to happen. There's actually a couple good reasons not to. One, it WILL cause some nasty ingrown hairs, as you're going to find out if you use it on your groin and lower abdomen, and two, the hair roots on your face are immensely responsible for keeping you from sagging and wrinkling.
I pull everything from the toes to my bra band line. Just couldn't handle the chest area. Prefer to shave that each night in the shower when I'm shaving my mug anyway.
Enjoy the experience. I'll be "rooting"
for you! Patti