Nipples. (And constructive criticism.)

I'm still unclear about what, if anything, man-nipples are able to feel.

As in a lot of sexual things, there's two confusing factors:
1) Bodies are bafflingly different. What makes one guy throw his head back and gasp makes another guy go "Uh, why are you touching that?", and apparently there's no predicting it.

2) It's hard to get an honest opinion. Because usually guys don't say "Why are you touching that?", they privately think "We're naked and she's touching me, I'm not gonna criticize." Which is probably for the best, but it does make it hard to improve my technique. I honestly have no idea if I'm good in bed--clearly I'm not terrible since I do okay in repeat business--but no guy is ever going to tell me the difference between "not bad" and "fucking fantastic."

So. Man titties. Numb to the world? Massively erogenous? Only as sensitive as ordinary skin? A little sensitive but nothing special? Individually variable and therefore an unknowable mystery because no one is rude/honest enough to communicate these things?

Maybe I should hand out Post-Coital Satisfaction Surveys to my partners. "On a scale of one to ten, please rate the following factors... cleanliness... taste... customer service... nipple stimulation..."