I agree with LukeB regarding 2312: It's a very enjoyable book, but probably not something to read if you're in the mood for space action, because barring just one or two specific chapters said space action is just not there.
I found that my initial feelings about how the characters were built didn't change much, specifically that the most central characters are very engaging and dynamic but that the secondary cast is somewhat blah, moreso than their secondary status justifies. They too often get reduced to talking heads, I found. The antagonists in particular are near-complete non-entities, and while I think this may be kind of the point [the banality of evil, etc] the way it is executed makes it feel as though the book just can't be bothered with them, largely because they are connected to the plot, which it can also not be moved to care much about.
The exploration of and immersion in Robinson's future solar system, though, ... well, dang. It's wonderfully detailed, and also nicely balanced, in that Robinson's vision both acknowledges potential future problems [some of them quite horrific] and also allows the future to be fun and awe-inspiring. [If awesome prose poetry about space is your thing I imagine you'll be very happy, because Robinson's got cosmosporn of the most brain-baking kind in heaping spadefuls doled out generously.] It's a big book, both physically and in its concerns, and in the rush to pursue these concerns and visions I just think it forgets about the little everyday mechanics of storytelling occasionally. Highly recommended, by me at least, but not as a brain break or action read. As LukeB said, I didn't find the book especially hard to read -- though I understood well under half of the science Robinson was talking about --, but it did demand my full attention, and didn't exhilarate in the same care-free way as, say, Leviathan Wakes.