Whoa, Where Is This Writing Going?

The Kentucky Derby this past weekend had me thinking about riding and writing. 
In the world of writing, there are those that plan and outline before the first draft and those that dive head first into the first version of a story. These days, I figure out my story ideas and get to know my characters as I outline plots before hacking out the first drafts. But whether a writer outlines and plans, or dives into a story, there are times when that story runs away with a writer. This is a lot like having a horse take a bit between its teeth and gallop off into oblivion with the writer on its back.
Photo credit: This lovely photo is by jade at www.morguefile.comSure, wild rides can be scary, but they can also be exhilarating. Runaway writing can offer new insights and revelations. A writer, like a rider on a rebellious horse, could end up in a better place if the writer doesn't fight to reign in or pull up the runaway. 
When my writing takes off on me, I remind myself that writing an outline doesn’t mean I can’t or won’t change it. The subconscious often knows best and sometimes takes the bit. Keeping an open mind and not being shackled to an outline or any preplanning can lead to a tangent that might be just a wild ride, or might also lead to a much better story. Why not drop the reigns and find out?
Have you had stories take off on you? If so, what have you learned from these wild rides? If you outline or prep, do you reign in a story that takes off or scrap the pre-planning and enjoy the ride?