As a generalisation,the first book of a series is the best one, with gradual drop off in quality as the sequels go on.
A case in point for me is Alistair Reynolds "Revelation Space" Series and John Scalzi's "Old Man's War". Although i am in no way saying that these sequels aren't worth reading, i loved them, but they just weren't up there with the first book in my opinion.
I read a lot of the earlier Star Wars books. They're okay, but not great SF. Something of a guilty pleasure of mine.
Dune is superb, but i could never get into any of Herbert's Sequels.
I think stand alone books are better, but i do like to return to the familiar comfort of a known universe.