If I had to pick only five manga series to read for the rest of my life, Mars would definitely be one of them. The best way I can describe this series is as a soap opera, what with mysterious deaths, unknown twins, and crazed murderers, but despite that unflattering description, it's an insanely addictive series that will keep you hooked!
Please be aware that this review contains some mild ***SPOILERS***! I've tried to keep them minimal and vague so as not to spoil everything, but I'm not really sure how to properly bubble about this series without mentioning a few of the details.
I really loved the characters in this series. Naturally Rei and Kira in particular, as they get the most development as the main protagonists, but it's really impressive to see such interesting, fleshed-out characters. They don't fit neatly into stereotypical character slots; Kira starts out as the shy artsy girl who was the victim of abuse, but you get to see her not only come out of her shell, but even have some traits that are rarely ever shown, like getting cranky when she's bored, or actually letting herself have feelings of vengeance and hate. The same goes for Rei: he starts out as the slightly misunderstood playboy, but even when he's very much in love with Kira, you see him doing normal guy things like checking out another girl's cleavage, or getting flustered at being misunderstood. I liked getting to watch them slowly fall in love and learn about each other without the usual plot that involves unrequited love from early childhood finally coming to bloom.
The supporting cast is also excellent. Tetsuya and Harumi are fantastic, even if Tetsuya can sometimes get stuck with some cheesy lines. And even though he's something of a minor character, I really love Rei's father. The mangaka really made you feel the heaviness that surrounds him and makes him behave and treat Rei the way he does.
There are so many twists and turns to this series, and as I mentioned before, it's almost soap opera-y in nature. And yet it doesn't come across as far overdramatic. Things unfold and are revealed at a really good pace.
This was one of the first series I ever started collecting, and I reread it at least once a year. No matter how many times I've read it, I still cry at the ending. EVERY DAMN TIME! It's so good!