I'm a reader that's fairly new to manga, so these reviews of mine should be read with that in mind.

On the surface, Rosario+Vampire is a lot of why I never got into Japanese manga & anime before; it's chock-full of clichés. The school setting, the unwanted harem, the monster-of-the-week common plot, etc etc. I was pointed to it by friends who told me "It's better than it sounds", and indeed it is. For one thing, it gets better with time. The art improves, the stories improve, and the characters improve.

The story does get darker from about half-way through this first run, and in my opinion it's a turn for the better. Tsukune undergoes significant character development because of it, as does "outer-Moka"; "inner-Moka" not so much in this first series (more so in Rosario+Vampire II)

There is a fairly large degree of fanservice in the manga, though I hear the anime has more. Most of the time, it's not enough to harm the story. It's noticeable that Moka tends to be treated with a little more respect by the artist than the other girls (perhaps because she's the main girl)