Well, I'm not into shoujo manga since almost every story I've read was really lame so I kinda give up on it for a while, until now. So, I decided I'd read Mars just because it has 400+ 10's and i am a manga fan (and had A LOT of free time at home 'cause of rainy cold days)
To begin with, I enjoyed reading a good part of it; art is old but actually well done, plot was classic (and I was prepared for it), and characters mostly predictable but quite defined, I liked their development through the story.
I liked the way they met, and like Kira says on the latter volumes "It's strange how people meet" (I nodded), that aside I think almost everyone knows someone who's similar to Rei in some ways, like I do, it was fun remembering that person along the manga, and I also believe that everyone in their life, meets a person who is capable of changing you and to change because of you in such a dramatic way. For me that's the greatest thing of this manga.
The rest of the story was good.
The only parts that annoyed me were the ones when Rei forced contradictions (or something like that) and those times they had to deal with Masao, he pissed me off.
Yeah it's a great example of shoujo manga, stupid, touches difficult points, entertaining, and worth reading (once).