I am a total ecchi-enthusiast, so keep in mind I might be a little in love with this show when writing my review.
Haha. Most people would probably shrug this show off as "Are you serious?" or "Wow, what perverts", as the synapsis makes it seem like Yamada is a total slut and that all you'll get is very intense fanservice for 12 episodes. Think KissxSis or Rosario+Vampire-esque. But B Gata H Kei is not purely fanservice, there is actually a very funny story, and when there is fanservice it's not that trashy, are-you-kidding-me kind. A girl who thinks her vagina looks weird and wants 100 sex friends? Sounds good to me. But there is so much more to B Gata H Kei. A girl's quest to have sex filled with hilarious dialogue and outrageous situations. Ecchi comedy at its finest.
The art is pretty good. Yamada and others are well drawn and cute and backgrounds and scenery (like Kanejou's house) can be very pretty. Kosuda definitely fits the generic high school male description, and this is very refreshing in contrast to the majority of "normal" anime males with six packs, spikey blonde hair, etc. Boobs can be a bit huge in this show, but it's not disgusting and trashy because of B Gata H Kei's comedic element.
The show has a decent amount of popular seiyuu including Tamura Yukari and Horie Yui as Yamada and Takeshita, respectively, so sound is nice and the voice acting isn't forced and unbelievable or anything. The OP and ED are cute to me, and will probably be cute to you too if you're a Yukarin fan :). The background music isn't made up of mellifluous symphonies or anything TENMON-esque, but it gets the job done and isn't annoying or anything. The random "B Gata H Kei~" is cute. ^^
YAMADA. I love Yamada so much, she'd get a 10/10 from me. Yamada is so entertaining, and her sorta tsuntsun side, though cliche, can make episodes very funny. The best part about Yamada is that she's daring, as you'll see if you watch the show. And let's face it, it's not often that you have a sex-obsessed female character in anime.
Kosuda can also have his awesome moments, though he usually fits the typical high school male role that he's supposed to fit (which is a good thing). Other characters like Takeshita, Kanejou, and Miyano often add to the comedic relief of the show and are valuable aspects to this show's comedy.
I wish I could give this show an infinity in enjoyment, because that's what really makes this show a 10/10, the only 10/10 I've ever given, for me. I remember watching every episode of this show and not wanting it to end. This show is hilarious and I enjoyed it to the fullest extent. Today, 6/18/10, I finished the series and I sort of wanted to cry, because there'd be no more of this show to entertain me on a weekly basis. ): The comedy in this show is amazing, though it can still manage to be serious and/or erotic.
B Gata H Kei is an amazing watch if you just want to enjoy yourself and don't mind a little ecchi. If you're an ecchi-enthusiast like I am, I highly recommend this show, as it is probably the best, and seriously the classiest ecchi I've ever seen. I know classy ecchi sounds a little bit strange, but I promise you, this is not a Rosario + Vampire kind of ecchi with panty shots galore, but the fanservice is there where it's supposed to be, the comedy balances it out so it's not gratuitous.
If you love ecchi and comedy, you'll surely love B Gata H Kei~!