GIVING UP THE GHOST is and always was a stand alone. It is the most important story I could have told about those characters, and nothing else I could imagine happening to them comes close.
POSSESSING LUCY was always intended to be the first in a trilogy/series/something involving more than one book. Lucy's got a lot of stuff to work through, and I don't think I could have done it all in one book if I'd wanted to.
THE HALFWAY HERO started out as a stand alone, but as I was writing I realized there was a lot more that could be done with the world and the characters, and their story doesn't really feel over yet to me.
Funny thing is, as much as I love the idea of writing a series, of getting to stay with and develop characters and plotlines over multiple books... I don't actually read series very often these days. Not because I don't like staying with characters over several books--mostly because I've found that the majority of the time I'm disappointed by second books. There are many many first-in-a-series/trilogy books that I read and loved, or at least liked a lot, only to have the next book leave me with little more than a 'meh'. So I've kind of stopped reading next books unless I either feel the first book was so good I can trust that the author will deliver something close in the second (which is not always the case, but I've had pretty good luck), or the first book wasn't amazing but I can see the potential for it growing into more as the series continues. Most of the time, I'd rather just enjoy that first book, and not tarnish it in my mind with less-worthy continuations. ;)
On the other hand, I loved series when I was a kid. I cannot tell you how many BABYSITTER'S CLUB or SADDLE CLUB or SWEET VALLEY books I read, because, well, there were an awful lot. And that's just the continuing series--I also devoured the Ramona books and the Indian In The Cupboard sequels, the Narnia series and the Time Quartet. When I was a preteen I moved on to Anne McCaffrey's books and THE DEATH GATE CYCLE and the like. If I liked a book, I immediately went looking for sequels. If there were none (or none more), I wished there was. Never enjoyed parting with characters.
I want to do both. I want to write those wonderful stand alone novel that do everything and say everything you could possibly want in one book, and I want to write the sort of series I was always searching for as a kid. Unfortunately, as far as I can tell, it is impossible to do both at the same time. But I'm sure I'll figure something out. :)
How about you? What's your stance on stand alone vs. series--as a reader? As a writer (if you are one)?