I read all three, and it was a struggle. I found the writing in Robinson's Mars series to be adequate in terms of mechanics, but the plot and characters were boring.
That's perhaps too harsh -- the characters themselves were okay, but what they did usually wasn't so interesting. The books lacked in tension.
And that is again perhaps too harsh -- the real problem, I think, is that he would spend 1/4 of a book on one character then switch to another, and what they did was totally unrelated and you would never go back to the first character.
The first book had a great lesson on what [b]not[/b] to do. He starts off in the "present" by showing how everyone reacts to the death of a particular character, then starts the rest of the book as flashbacks. The character who died becomes a main character, in fact THE main character, in fact the only truly interesting character. So you already know he is going to die, and basically how. There is no tension. When the flashbacks are over and I got to the part where he died, I lost interest completely. I had to force myself to read the rest of the book.
In fact, I had to force myself to read the rest of the series. The series is a showcase for Robinson's vision on colonization of Mars and the rest of the solar system, and major technological and social developments in the near future.
It's really, truly, brilliant stuff, and it's worth reading just for that.
But it's tough going. I felt enlightened, and relieved when I finished the third book.