I'll be the first to admit - back in high school and college, when we'd have to sit down and write the outlines for our papers, I'd roll my eyes and fight it. HARD. I mean, I wanted to WRITE. What the hell does outlining have to do with writing?
These days I've kind of matured - in part thanks to a screenwriting class I took through UCLA. In that class we wrote what they called a step outline of the script - two sentences, scene by scene, outlining the story. At first I hated it. But I do have to admit - it's got some advantages.
Writing this way (I'll usually write a sentence or two for each chapter) forces you to think through the story. There's fun in flying seat-of-your-pants style, but I think that's better left for shorter pieces. When you're dealing with a longer piece, there are a lot of things you need to keep straight.
I'll also somewhat sheepishly admit that when I wrote THE TRIP, I didn't have a full formal outline. I would outline a few chapters ahead of where I was, so I had an idea where I was going with the story. I got a little of the fun of exploring the story without feeling locked into anything as I was writing. (Seeing as I was doing this for fun at the time, no harm, no foul)