Blue Mars
Overall, this was probably my second favorite of the "Color Mars"  Trilogy.  I was very slow to get into it.  However, by this point  Robinson has refined almost all of his main characters, making them all  very interesting and flesh-out, as opposed to the one-dimensional  cyphers they were in 
Red Mars.  I even liked Ann and Maya this time round, though it took me until almost the end of the book for the former.  
The  only viewpoint character I didn't really like was Jackie's daughter Zo;  she was definitely the worst viewpoint character across all three  books.  Hiroko was actually bearable this time around, mostly because  she spent the entire book missing.  This was good, because I'd never  really bought Hiroko as a character; she was always felt unmotivated to  me, and I could never figure out why she was held in such high esteem; I  personally just felt she was weird.  Unfortunately Nirgal, Art, and  Nadia draw the short-straw here after some wonderful fleshing out in 
Green Mars; the latter two's character arc is just kinda dropped and never concluded.
Throughout most of the book, my real problem was that I didn't really know what it was 
about.  
Red was about the colonizing and first revolution; 
Green was about the second revolution.  Though 
Blue  was initially about building a new Martian government (and there was  some neat stuff here), the middle of the book felt pretty aimless to me.   The end introduced the immigration crisis, but it was at this time I  realized what the book was actually about.
Unlike the first two, 
Blue Mars  is not really about the history of Mars.  This novel is about long  life, and the problems of living the centuries that the First Hundred  can.  Michel returning home to Provencal was one of the best parts of  the novel; the looking-back provided was wonderful.  Maya coping with  her increasing distance from her own past (a plotline introduced in 
Green)  was also very interesting.  Sax's determination to solve this problem  gave a strong focus to the end of the novel, because we saw through him  and through Maya just how serious this memory problem was.
The  best part of the problem was, well... the solution.  The scene where Sax  summons the remaining Hundred to Underhill, and they take the treatment  was downright wonderful.  If any part of the book was going to move me  to tears, this was it.  Sax's almost stream-of-consciousness rush of  memories was extraordinary.  The ending was also great in that it  finally restored the bond the Hundred had had during the journey on the 
Ares but lost since.  As the memories poured in, for the first time in centuries, the Hundred 
understood  one another once more, and lost the distance that had been created  between them.  Sax and Ann finally figuring out why they were  antagonists was great; as were the two brief bits with them rock  climbing and sailing.  (By the by, Sax's boat is 
almost as cool as the Zygote boulder-cars.  Almost.)
Though I have had much praise for the book here, I would still place it second beneath 
Green Mars  because of its lack of a strong plotline (the memory thing and coping  with being so old is more of a theme until the very end) but above the  plodding story and one-dimensional characters of 
Red Mars.