On the one hand, I love this series. Robinson's recurring characters, the survivors of the "First Hundred" and their offspring, are memorable and fascinating. Another fascinating aspect of this story is the ever-unfolding terraforming of Mars. We start from the bare-minimum survivability achieved at the end of Green Mars, and eventually move on to seeing bees, sequoia trees, and even polar bears (albeit genetically altered to survive a thinner atmosphere).
Also, we "see" (believe me, Robinson's writing can do this) the changing of Vastitas Borealis into a Northern Sea; a channel burned into the surface to release volatiles turned into an actual, open-air canal; tented cities becoming seaside resorts; and the pink and brown sky gradually shift to an actual, Earthlike "sky blue." Out beyond Mars, the asteroids and outer moons are being colonized as a means of relieving population or prison pressures on Earth. And Earth, suffering from the flooding created by the Antarctic Ice Sheet, has space elevators that are so massive they have multiple tether points on the surface. This is world-building writ large, and Robinson makes it all seem marvelous and believable.
Naturally, that's just the technological angle. Politically, Mars is in the process of creating a world constitution. (You can read the full text of this constitution in The Martians.) There are some points about the constitution I don't like, mostly its emphasis upon the judiciary--particularly the environmental and other courts--to become the primary arbiter of power. There are some good things to like in the Martian Constitution system as well (like the "Australian ballot system"), but that's a talk for another day. Suffice to say, once the constitution is formed, life on Mars goes on, in semi-peaceful, matriarchal, environmentally-sensitive fashion.
The Martian matriarchy begins to export its technological products off-world, and in the process extending its political power. Jackie Boone's daughter Zoe (or Zo) is one of the primary matriarchs, and she is completely ruthless in her tactics. The still-ancient Ann Clayborne rightly calls her a "thug." Back on Mars, Jackie has become a power.
The First Hundred themselves are getting old, old, old. They're losing their memories, facing problems not curable by their gerontological treatments, and generally becoming strange. Old Sax Russell is still on hand, however, to apply his relentless intellect to their memory problems. This is where the book starts to slow down. How much information does a reader really need about the chemical process of aging? Other passages get old fast, especially if you're not interested in or an expert on genetic engineering or rock formations.
One cute bit in Blue Mars is the story's connection to the world portrayed in Robinson's The Memory of Whiteness. Obviously, KSR is attempting to make many or all of his stories into one comprehensive narrative. There are some continuity gaps, but you get the picture. Blue Mars completes the cycle of the series, and probably had to be written. However, the first two books make the best points and are much more fun to read.