My opinion? That depends. What sort of hentai are you talking about, and which titles?
For anime...a few titles have some redeeming value, either as storytelling vehicles or straight-up porn. Good titles for story include Pure Mail, Campus, Koihime, Mei King, Elven Bride, Viper GTS, Wordsworth (supposedly), and to a small extent Hatsu Inu. Sex Friend and Resort Boin are okay as porn (Sex Friend feels like it was trying for a story, but opted for more sex).
Going beyond these titles...be very, very careful. I can name far more pure dreck than I can diamonds. >_<
For eroge...might I direct you to some of my reviews? Eroge visual novels have become sort of my specialty, and I've run into some <url=http://www.escapistmagazine.com/forums/read/326.135672>truly fantastic ones, <url=http://www.escapistmagazine.com/forums/read/326.146260>decent ones, <url=http://www.escapistmagazine.com/forums/read/326.148564>questionable ones, and <url=http://www.escapistmagazine.com/forums/read/326.130703>horrible ones. Mercifully, I haven't played any truly horrific ones, because I tend to filter them beforehand.
For ero-doujinshi...I've come across a few gems, and a mountain of dross. It'd be much easier to list the stuff that's good (try Aqua Bless, Virgin Night, and Yuria Shiki...though that one's not explicit at all) than try listing the stuff that's mediocre at best, nauseating at worst.