This may just be me, but my nipples are almost numb. I can feel it when they're forcefully bitten or pinched (mmm), but trying to gently caress them? Doesn't do much. My breasts can appreciate soft touches; although the nipples are pink and sticky-outy and all, they're actually not sensitive at all.
I still like seeing a dude's mouth on them though.
Your breastage may vary. But it's always worth asking! It's pretty widely known that some men have sensitive nipples and some don't; I'm here to point out that women are the same.