Series or Stand Alone?

As a writer, I'm always looking for a better grasp of reader expectations and preferences. What's your favorite? Do you like stand alone titles or do you like to revisit the same characters?

Mystery or thriller authors are blessed in that they can create a main character who will go the distance in multiple titles. Romance readers expect some sort of closure for the relationship by the end of the story so it's difficult to string the same characters into multiple books with the same level of tension. However some authors have done it, most notably Diana Gabaldon, JD Robb and Stephanie Meyer.

Other authors write long open-ended series to create 'related' titles. They devise elaborate family trees and reach out to use cousins and distant relations once the large clan of siblings is suitable wed. You don't have to have read the others to enjoy one, but it helps.

Then there are books that create a discreet world within their pages and once finished, you never see them elsewhere. Hopefully because the ending is so intensely satisfying, going any further would diminish the experience, not add to it.

As a reader, what do you prefer?