I tend not to like long series, as I feel the authors often run out of ideas after a couple of books. I understand the appeal-- people want to hear more about their favorite characters, or find out "what happens next" in their favorite world, but often the author has already told the most interesting or original story in that setting or with those characters, so once the original book or trilogy or whatever is done, everything after that feels like a real stretch.
Sometimes authors do find new life in old books later, though. In particular, I liked Ursula Le Guin's
The Other Wind, and felt that in many ways it was stronger than the original Earthsea Trilogy. And I got tired of the later Pern books, but then I read
The Masterharper of Pern and it was like old times again.
Most of the authors I read and re-read have been mentioned here (classics like Heinlein & older McCaffrey, e.g., plus many of Ian Banks' Culture novels), but I'd mention Garth Nix, Charles de Lint, Shannon Hale, and Tamora Pierce as more current authors that I follow closely. I don't draw so much of a line between science fiction and fantasy in my preferences, but I tend to avoid "high fantasy" as so much of it tends to be Tolkein knock-offs. I know a lot of people like that sort of thing, but most of the time I'd rather just read Tolkein, or read something really different and original. I tend the feel the same way about "space opera" or even "military sf."
I think it's like music -- tastes are very personal, and if you really like a genre, it's easy to tell, say, Mozart from Vivaldi, but to some people it all just sounds the same.
