Cities in flight is actually four books, that were written quite far apart, and out of sequence. Blish uses a small number of crucial characters in the rise and fall of a Terran galactic empire, apparently based on the excellent "Rise and decline of the West", by Spengler.
The characters are interesting, and involving, though not very consistant or credible. Blish's major failing is that sometimes his situations are very very derived and incredible, to the point that you wonder "Why did they do that" ?
The overall feeling of the series is one of ineviibility - you can see the decline of empire after empire, though they only view of the empire itself is through the lives of a small number of people. Though I did enjoy reading it, many of the ideas that Blish has shoe-horned into the plot are strange and dated...or maybe just innovative, but from a different time.
The SF Masterworks series is an excellent cross section of SciFi books. This is a masterwork in it's own way, but like many of the series, I can see it missing the mark with some people. If you like SciFi - buy it. If you just like some scifi books, don't.