I prefer series because I think it gives the story more depth. However, like many others have said, if the series goes on for too long, people lose interest because it becomes too diluted with repeating events. That doesn't mean I won't read standalone stories though. I think it all boils down to a quality over quantity kind of thing. A standalone can be better than a series if it is very well written, though i'm guessing that it would be easier to create more depth in a series than in a standalone story.
For series, I tend to prefer trilogies. Such as the Matrix, Bourne, and the LOTR trilogies. I think it also helps when an author writes many stories that are based within the same universe, but can be read independently.