It's as good/bad as any other media format without an unbiased frame of view, the same way, westerns, slashers, action, lifetime, ect... movies can, alone, portray positive/negative messages.
There are quite a few things, in which are rather common place that do not accurately convey real life. To a child, who does not know what hentai is,(for the greater part) fantasy, they could easily misinterpret messages or underlining themes as true/real.
For instance, at least half(40-55%) of hentai is rape, and most often end with the rape becoming consensual(95%), at some point or another, even fewer times, do the perpetrator end up penalized for their actions(0.05%). To a child who infers hentai as an "accurate portrayal of life", that can be bad.
There is a more subliminal message of, "every woman around men should be a possible 'sexual conquest', who, with enough wooing, will put out sexually, expecting nothing in return". A message porn, action, slashers, ect movies conveys as well, but hentai in a more apparent and raw manner.
Long story short: I don't believe in good or bad, with them being subjective, so if you want one, it's difficult. They have good and bad qualities no different than any other media format, IMO, and ultimately relies on all the other knowledge, experience, and intelligence present during any scenario involving media formats.
Now if you want personal feelings towards it, I love hentai. I love the art, both in the best and the worst of times. From cliche and over used stories with a different art style to those rare gems that make you feel all the wiser for having experience and in which you can reflect on with a warm experience. To view the imagination of a man/group, when given an unrestrained opportunity to tell a story they wish to convey, make such an enthralling and immersive story/world/characters that you forget to fap the first time, to impatient to get to the next page and find out what happens next. The stories that allow you to just fantasies and dream what you can never truly experience in a little bit more vivid manner. The bond found between the communities that share in those fantasies and dreams, and allows them to share and strengthen each others'.
(I don't have a stomach for every type of hentai, but I would never censor anyone.)