Lilith, April and Faith are all girls who I've experianced nipple sucking with and the more I enjoyed it, the harder they sucked and bit on the suckers. I think it is as much a turn on for them as it is for me. Similar to when I enjoy sucking a nice pair of nipples and listening to my SP moan.
Little known fact that mens nipples can be just as sensitive as a females. And like Gordd54 mentioned, when it's done correctly in the midst of a HJ or fingering of a gal, it just heightens the orgasm.
Beware of Miss Lilith though. She might just get the urge to put more than her lips on your nipples. LOL.