I have always written more or less "by the seat of my pants." This method allows for inspiration and immediate gratification, but lacks planning and foresight and can lead to an awful lot of rewriting that could have been avoided. Most published authors extoll the virtues of outlining, and I add my voice to the chorus proclaiming it to be the best method for ensuring that your novel or other writing project will end up closer to the way you envisioned it than if you did not employ an outline.
Outlining can be an enjoyable process, but it can also become all-consuming. However, as the outline process progressed, I noticed that the outline was becoming more and more detailed. So detailed, in fact, that dialogue and scenes were beginning to clog the outline and make it unwieldy. It became difficult to use as the broad overview it was envisioned to be. That is when I decided to begin writing the novel as I continued the outline.
I am currently snug and ensconced in my tree, writing my new novel while still assembling a detailed outline. This is not advisable, but so far it's working for me. I would prefer to completely finish my outline before beginning the actual writing process, but I was eager to begin and sentences, descriptions, and dialogue were tumbling around in my head. I felt the best way to proceed was to begin the writing, even though I know I will have to change some things. Since I know how this story will end (unless it changes, as endings often do), I have no problem with writing while outlining. Time will tell if this method works.