I agree with the consensus here. In SF/fantasy particularly, the reader is going to be plunged into a world where things aren't the same as this one. If it's an actual alternate world (other planet, future society, magical kingdom, changed history) there will be the experience of being plunked down in a foreign land where (somehow) one knows the language but otherwise have no clue what it's about, but also have limited access to at least some people's heads who do know what it's about. Understanding the world should, IMO, be like putting a puzzle together. Too much up front spoils that experience. At the same time, delivering comprehension too slowly creates ongoing confusion.
One thing that I sometimes do is write a prologue or backstory with no intention of publishing it as part of the book. That way, I have all the facts down for myself. (Because really, one often starts a story with only a rather vague concept of what the world is like in which it takes place. Or at least I do.) In writing this ugly beast, I often stimulate myself to plot ideas or new characters. Some of it may occasionally be good enough to chop up and insert at appropriate points in the story, but do this sparingly. The problem with info dumps is not just that they're long and boring (though that's often true as well), but that they break immersion and remind the reader that he's a foreigner.
