Refreshing take on a tired harem genre.
Maria Holic can be summed up many ways, but I prefer to use the a food metaphor and say that it's light, airy, and a little sweet too.
What really makes Maria Holic one of the most refreshing anime I've seen is that it doesn't bother with trying to be something it isn't. Many anime series' try very hard to be everything at once, with love arcs, action scenes, murder mysteries, etc. all thrown in to the main plot. The best thing i can say about Maria Holic is that it certainly doesn't overextend itself, stays true to the characters and original theme of the show throughout, and maintains itself as a funny adventure into someone's life.
I loved the art style and voice acting. Creative direction on the show itself was top notch. The art style was very cute and appropriate for the show. The voices were all very fitting of their characters as well. In particular, Mariya's voice actor did a great job and shows great control of her voice during delicate scenes.
The show was so refreshing, in that it took the perversion out of a show that would be otherwise ripe with it. It doesn't have gratuitous panty shots or explicit references. The show itself behaves in a way that is foreign to most anime - it actually vaguely respects the female characters within it. Every character feels like a developed person, not just an archetypical, "tsundere," or "moe". It's nice to actually watch something that treats it's female characters as 3-Dimensional while at the same time tackling the issue of sexuality. I feel that I liked the characters more this way. I thought that they were all adorable and unique in their own way.
The problems I do have with the anime are nitpicky at best. Maria Holic loves to do art-style changes in the blink of an eye - I found some of the art in these transitions to be kind of bad, and they were also too sudden sometimes. I think that some characters could have been more developed compared to others, Shizu and Yuzuru being two prime examples. Sometimes there would be fourth-wall breaking sequences that happened too often. They started happening late in the first season, initially as small jokes, but then they extended into regular happenings and they detracted a little from the storyline.
Overall, I think it's great, underrated and that more people should watch it, ecchi-hounds excepted.