I do enjoy the occasional series. They should be limited to about three books though. The author has usually exhausted most of the original concepts and ideas by then. An example would be
The Name of the Wind, by Patrick Rothfuss. I have been eagerly awaiting the third and final book for 10 years now. (This might also be an argument against series.
)
The exception might be
Discworld by Terry Pratchett, because, well, Discworld.