I am going to be perfectly honest with you. I have never in my life seen such a beautiful hentai, if it can even be considered one! I was sick and tired of overly sexualised females making out with other people in a completely implausible way. You can tell an actual story without going to the cliches like "big breasted sister has sex with shy brother" or "naughty school teacher has her way with a rebellious teenager". So humane, so gritty that the passion the characters feel for each other actually means something! I read the comic of a lesbian who loved bondage, and I was blown away by just how expressive and endearing the characters were. It really puts every other hentai story I`ve read to an absolute shame.
Hentai is strictly Japanese-Manga-style erotica- this would be considered just a porn comic, except you're right. It's not porn, not really. Hell, it's not something for kids to read, but it's a beautifully artistically rendered story about people, their friendships, their sexuality, and how it all fits together. It's not about over-the-top depictions of sex that are nothing like real life, it's an emotional and realistic portrayal of wonderful characters. The fact that almost every straight male who reads this is gonna get a woodie is beside the point.
According to someone I knew that had a Prince Albert and nipple rings, yes, it does increase sensitivity. And pleasure, and a few other things. (No, I don't know specifics. Wasn't my place to ask. -_^)
I think at some point in this series, the topic of limits should be brought up. BDSM is a many-faceted thing, and not all doms and subs are into every aspect of it. In particular, what one sub may be fine with or even enjoy, another may very well not, for any number of reasons (I'm reminded of Marion and how upon first meeting Ally, she told her in no uncertain terms that even though she's a sub, she's not going to roll over for a threesome). I bring this up because BDSM can be quite intimidating for newbies, especially those whose only previous exposure to the scene was media and porn, and talking about and knowing one's hard and soft limits and what one may and may not have done to them goes quite some way toward overcoming that initial reticence -- I daresay probably even more so than having a safeword.