I didn't learn much from reading hentai but I did learn a lot from watching hentai, like "kimochi ii-desuka?" (does it feel good?) or like "oppai misete!" (let me see yer boobs") and other stuff like those... ^^
I got the grammar out of a quick study, and most of my vocabulary came from puzzling words out using grammar rules and the few simple words that I knew at the time.
I do plan to get around to formally studying, but either I'm lazy, or busy.
I'm picking up words from watching anime,
learning Katakana (this stuff カタカナ) from translating stuff like options in games and websites,
and I just started to try and learn Hiragana (this impossible to read crap ã²ã‚‰ãŒãª) from playing and trying to translate the Haruhi strip Mahjong game Teclo posted
but I would think it to be very hard (perhaps near impossible) to learn Kanji
(I cannot describe with words my hate for these æ¼¢å—)
with its 50,000+ different characters
Learning japanese can be pretty handy, but I don't think reading hentai can teach you that much. Besides, I would much rather be able to speak it then read it, seeing as how Japanese has some thousand letters.
I studied japanese...8-9hrs a day at home private tutor...6 times a week...for 2 1/2 months coz they said id be working in japan...but unfortunately i was sent in NY...fck that...
It was really hard, i started with hiragana then katakana in the morning fck kanji i didnt study it, then memorize 2 pages of words and their meanings, then construct sentences...But i gotta say...Watching Anime from my younger years really help...Greetings were easily remembered, the past present and future to negative words were made easier or i should say remembered...
Anyway...Yes...Watching anime, from different kind of genres...Hentai...Helped me alot not actually learned...and to conclude...i forgot about everything i learned about it, it was 2 years ago....lol