I was disappointed. When I first started watching it, I actually had the title mixed up with another I had seen - so I was completely unfamiliar with the plot.
But, in spite of the fact that there were people living on Mars, I feel like so much of the story had holes that made it unbelievable. I had so many unanswered questions... like did they just happen to pack an ultrasound machine on the spaceship? It's totally possible to know a woman is pregnant without an ultrasound. How in the world did Gardener survive without his mom? I doubt they packed formula, because that whole thing was not foreseen! Diapers? - even cloth ones? It's not like Walmart is down the street.
So, I suppose from the beginning I was made a skeptic.
I also felt like Gardener's character was not consistent. On Mars he seemed cool, innovative, he interacted with people and was geeky enough to be familiar with history on Earth (i.e. the story about Edison while in the classroom). But, then he's shocked to see a man on a horse (what is that?!).
It felt like a couple of teenagers with a whole lot of money sat down one day and said something like, "Hey, wouldn't it be funny if we made a movie about a kid who was actually born on Mars and then he falls in love and comes back to Earth and he's just super awkward?!"