I always write an outline. I flounder without one. However, when I talk about outlines, I'm not usually talking about a blow by blow. I just need a basic road map to give me a starting point to launch from and an "end state" to reach. All in all, my outlines are very basic in comparison to the finished product. I've turned entries like this:

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2. Derek POV. Derek has a nightmare about his dad's murder / combine this with an image of Gary Clark shooting Derek.
and this:

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6. Meredith POV. Derek arrives home from the hospital. Continue laying groundwork for addiction. Carolyn visits at the end.
into 10k and 30k word chapters, respectively.

I use my outlines only as guidelines, not hard rules. I never shy away from letting inspiration take me where I want to go at the time I'm writing a particular piece, but I always make sure I re-tailor my outline to fit new developments before continuing too far. Having a known endpoint and a way to reach it always helps me if I happen to get stuck. I've never had an unfinished story since I switched from pure impulse to my flexible outline method. (I know, I know; knock on wood, right? smile

Grant me the serenity to accept the things I cannot change,
The courage to change the things I can,
And the wisdom to know the difference.