Usually I'd start talking scores, but first I want to talk story differences
I don't want to give spoilers in my reviews, beyond what is kind of necessary, so let's beat around the bush and state that our main character, Shuusuke, doesn't end up with someone in the anime, whilst he does in the manga. And this is disappointing to say the least, as the ending of the manga was very nice... sure, making Shuusuke a wizard is mean, but it was done with love.
In addition to this, the Yatagai sisters, which don't appear in the manga, are quite lame. They do not stand up to the Hozumi or Matsushiro-sensei story-arcs, and don't really fit with the theme of the anime or manga all that well either.
Aside from the last few episodes though, the anime story does stay true to the manga up until the point it started to be produced, which was around 1/3 of the way through manga, and the story it does stay true to is portrayed very well, deviating a little from the manga, but not enough to make you think it's a different story.
The art is nothing amazing, but it is unique
The art style for Onisuki is quite unique, whilst not standing out from the crowd. From the proportions of the characters, to their general expressions, you can tell this apart from other anime of the same genre with great ease, even with similar looking characters.
However, the background art, is very generic, and the scenery used is nothing amazing, so it isn't really worth much more than a 5 for the art score, but the fact that the characters look unique pushes just that little bit higher.
The sound is as average as you can get
Apart from a pretty good OP and an alright ED, Onisuki has a very average musical accompaniment. Though the music isn't amazing, it does not matter as the music is just there to fill the silence. What you hear isn't bad, and by no means is it distracting... it could just be a little better in places.
The good, the bad, and the ugly
The characters are where my opinion of the Onisuki anime dropped tremendously. In the manga, the progression of Nao and Iroha is enjoyable to read. The daily happenings of Shuusuke are funny, and the interactions he has with female characters is either hilarious or cringe-worthy.
But the anime doesn't do much in the way of character progression like the manga. Shuusuke is still useless around women, but Nao and Iroha seem to have a completely different dynamic, and it doesn't play well into the picture you have of them reading the manga - even if you only read up to where the anime and manga share the same story.
Nao honestly isn't too bad in the anime, but not as good as in the manga. Iroha on the otherhand isn't as close to her manga character's personality, and it doesn't really help a disappointing anime.
Honestly, I think if I hadn't read the manga first, this may have been a 6, but it can't scrape that after what it did to a story I enjoyed reading so much.
With that, I will leave you to enjoy the rest of your day. After writing this, I am contemplating adding a manga review section to this site, as I have read a few good manga that never got an anime adaption, and I think it would also be quite interesting to write some reviews for manga and anime, and see what I have to say about each.
I'm also looking at creating a comment section for the site, but that's a little way off. So for now, look forward to the Comic Girls review some time soon. I'm currently watching Konosuba, which may be the next review after Comic Girls, who knows.
 He doesn't actually get turned into a wizard, read the manga... It's a great read, and then you'll understand the reference.