The original plot of the video game is actually pretty decent, but the manga...The manga adaption emphasized the Disney aspect of the story than the actual plot, as it seems. Sora and his buddies are drawn in an almost-cartoon style, and the Organization looks more like a bunch of people with hearts pretending not to have them then a group of Nobodies willing to do anything for a heart. The manga also highlighted the cute/funny parts, and glossed over much of the little character development for Sora and the gang.
Another thing is that after Sora got a character upgrade from pubescent trying to be 14 (Seriously, Sora in KH1 is supposed to be 14!) into an actual 15 year old guy. That doesn't really happen in the manga, so Sora is stuck looking like an idiot, despite his costume change.
BUT...The beginning "Roxas" arc is the most redeeming quality of the manga. For one thing, Roxas isn't drawn like a doodle and doesn't joke every thirty seconds. Another point is that this arc is made many viewers quite emotional, even more so than the original video game did. Shiro doesn't take KH very seriously, but he isn't without talent.
Basically: If your a fan of the series for its deepness and complexity while managing to have Disney in it, especially the more recent games, the manga won't suit you, but at least read the "Roxas" arc. If you like the series because it provides an opportunity to play/ally/defeat various Disney characters, than you'll like the manga.
