For the Walker Saga, I kind of approached it as if there was a huge 650,000 word story to tell, of which I needed to split into seven books. Therefore I have cliffhangers. There are no definitive conclusions until the end. I just keep hoping I can get each book out reasonably fast so that people don't have to hate me and my cliffhangers for too long. :)
In some ways I always appreciated that Harry Potter (one example) ended satisfactorily in each book. They weren't exactly stand alone, but each had its own drama which was concluded at the end of the school year. Of course there was an overall storyline that continued throughout, but generally there were no cliffhangers.
I suppose I could have written in this manner. Each of my books are basically set on one of First World's youngling planets, and there will be drama involving each planets inhabitants, as well as the overall Abby/Seventine storyline. Oh well, too late now ;).