Novellas have made a bit of a comeback in recent years. Back in the early days of speculative fiction, many pulp magazines included one novella, or at least serialized one over three or four issues. I've found that novellas are becoming more common these days in the ebook arena. You get a reasonable story with enough headroom for worldbuilding and decent character development. I've seen this mostly in the horror and dark fiction genres, but a visit to the ebook store will show a surprising amount of newer novella titles to choose from.
I'm a fast reader, so a novella is like a good hour or two read when I want something entertaining but doesn't require days to slog through. A series of novellas can be grouped into a good long book if you enjoy dead tree format reading. When I sell books at conventions, people tend to gravitate towards the thicker books, as though they were buying words by the pound. Thinner books are not as popular unless they're particularly inexpensive. I have some novellas that I reprinted from the very early science fiction days and I only make a buck a copy. They're cheap enough to introduce young readers to some classic sci-fi. Otherwise, I stack novella series into large trade paperbacks with lots of colorful art on the cover.
When it comes to writing novellas, I consider them like short novels. I use the same novel structure but try to set up each act like a short story that builds upon the last. I've had a lot of success with several pen names over the years using this method. It remonds me of the Lester Dent method of writing pulp, except it repeats three times, once for each act.