I finished Cities in Flight this morning. I wasn't crazy about The Triumph of Time, the final volume. It felt like an afterthought. The saga of spindizzy New York and Mayor Amalfi pretty much ended with the conclusion of Earthman, Come Home, and having the characters suddenly have to tackle the end of the entire universe, and be the only ones in the universe in a position to do anything about it, however tenuously, was out of left field and kind of an implausible aggrandizement of the characters. The whole thing seemed rather unfocused, really, and I wasn't crazy about where some of the characters' journeys took them.
Also, the physics was rather ludicrous. It's disturbing to realize that this nonsense represented (or at least approximated) the best scientific models of the universe that existed as recently as half a century ago. Well, I mean, as a person I'm excited to see what great progress we've made in understanding our universe in such a brief span of time, but as an SF writer it makes me wonder: will my best efforts at plausible science fiction be this dated and ridiculous fifty years from now?
Overall, throughout the series, Blish never really put any effort into depicting aliens. A few alien species, mainly the Vegans, were discussed here and there and occasionally emerged as actors in the story, but they were always unseen, off-camera, and never developed in any degree. So the attempt to establish a new alien menace, the Web of Hercules, in The Triumph of Time is superficial and unsatisfying. The Web is presented as a rival to the main characters in their pursuit of their (literally) ultimate goal, but when they actually arrive and begin their attack, they're defeated with -- and I say this with no exaggeration whatever -- the press of a single button. The main danger in the novel, and they're an afterthought disposed of as casually as you could imagine. The only other real action in the novel comes from a clash with a band of religious fanatics who briefly conquer the planet, and while their defeat takes rather more time, it still comes far too easily.
All told, I can't say I enjoyed Cities in Flight too much. The first book (in story order, but the second one written) was pretty good, but it was just a prologue to the saga that gives the series its name, and I had my problems with all three volumes in that saga. And its ending was particularly weak, all the more so in contrast to the cosmic scale it strives for.