This manga has to be one of the best shoujo mangas I have ever read. In terms of my personal rank, this is definitely number 2 on my list (Skip Beat being number 1 of course ^^).
Story: The story of Mars is by far one of the most intricate and complex of which I have ever seen. You pulled into the world of pain and darkness of the characters but at the same time, you see hope and a will to live and overcome their pasts. I was totally drawn into the story. Some parts I was giddy with joy while other parts I was ready to cry as my emotions went on a roller coaster ride.
Art: Absolutely beautiful art by the mangaka. Although a little too shoujo-esque, the characters were beautiful nonetheless. There wasn't too much attention to detail, although its not always necessary. The art was fairly cute at some parts.
Characters: One of the best parts of the manga was how well the characters were developed. This manga delves deeply into the darkness and cruelty of human nature. The characters were very well fleshed out and we get to learn about their dark pasts and how they struggle to overcome it.
Overall, this manga has had an amazing impact and I would recommend this to any shoujo lover. Although not a light-hearted manga, I was able to fully enjoy this manga and its perspective on human nature.
Damn, I wanted to write this one... well, to late.
Yes, you are right when it comes to "Mars" being the best what shoujo mangas can offer. Even by having some stupid concepts it surprisingly coherent.
Please, don't call "Skip Beat" a shojo. I know by definition it fits into the genre but no... just no. It's just not right.