In the last few years I've got into series again: Stephen Baxter (Ring cycle, including the Destiny's Children series which feeds back into Ring), David Weber (Honor Harrington series), Alastair Reynolds (Revelation Space series), Orson Scott Card (Homecoming saga), Gordon Dickson (Dorsai), etc.
What was the problem with series? I thought that authors might be writing more and more books just for the money, and that the books would become repetitive. I also think cinema has given the sequel a bad name.
I now think that most of these concerns were simply wrong, or at least overstated, or only apply to some unusually poor examples of the genre. I have to say I enjoyed and still enjoy the sense of satisfaction at a good book, with a good ending, and the fact that this really is the last contact I will have with a particular universe does seem to make that brief contact more meaningful. There are also some books in series that I wish had been stand-alone: in particular Coalescence far outshines its sequels.
However I've rediscovered some of the great things about series too - they can tell epic storylines that don't easily fit into a single novel, or tell different but consecutive stories within the same universe; they can focus in on one character or location from a previous novel or take the storyline in unexpected directions. Despite what I wrote above, getting to know a good, interesting character and following their further adventures and development is also satisfying. I think many authors continue writing series because they themselves love the scenarios and characters, and a series also lives or dies depending on whether it is enjoyable to a group of readers. A good author is able to give a structure to a series of books - and is still not afraid to conclude them.