Ambiguity is always fun in writing. Getting the balance right is hard though.
Make it too obvious, give too much away and the reader is left feeling disappointed. Make it too obscure, and the reader is left confused.
Of course, this also depends on what you are trying to achieve. Do you want it unclear, or do you want it resolved, but with an edge of uncertainty. I mean, once the beast is slain, do you want it unclear as to whether the beast really killed everyone or if it was one of the protagonists... or do you want the beast slain, but a flash of something dark in one of the protagonist's eyes, the beast lives on in them?
Hmm... that wasn't entirely clear was it. It may be even harder to write about writing ambiguously than the ambiguous writing itself.
So in This Bedtime Story are Audrey's devils real, or are they just night terrors she has conjured up over missing her father? Obviously, I like the weird, so I'd like to think their is more to this world than most of us get to experience. If you know my writing you'll probably er on the side of the weird being the truth. But then, maybe those are just the nightmares of a child with imagination, the early stages of a writer like me.
I tend to try for the less obvious edge of ambiguity. I guess I'd rather leave someone confused than disappointed. Certainly in the past I know I've left people more in a state of WTF than I might have intended. But I would rather credit readers with intelligence than think they need everything spelled out to them.
There is danger in overworking it too. Less so in micro-fiction maybe. The X-Files, for example, was better when it was just It's aliens! No it's the government! when it carried on a few seasons with aliens/ government/ aliens/ government/ aliens/ government ad nauseum, I got a bit bored.
Ideally, obviously, you want to hit that sweet spot. Where the reader thinks they know what the truth is, but there is still a delicious edge of what if?, a splash of maybe.
There's a distinction to be made between ambiguity and suspense too. Or maybe a demonstration of symbiosis. And I think I've really been talking about something between the two. Because ambiguity covers a whole lot more than just a suspenseful ending and a lingering sense of unfinished business, although it is a great tool for that kind of effect.
Sometimes, ambiguity can just be bad or unclear writing. But when it is intentional and controlled I think it is something brilliant and interesting. Ambiguity is the opening of a discussion between the reader and the writer. What do you think is happening here?