I'm curious of this myself...I'm not at the point where it's really useful knowledge for me in the short term, but the prospect of hurting my partner, even in the slightest...sorta scares me.

Skitty, what I've heard is that it varies. I've heard it described as not painful whatsoever to...well, not agony or anything, but smarts a good deal. Generally, I've heard it described as a "pinch" of sorts. This is only what I've heard though, and as a guy, I don't particularly have any first hand knowledge there.