When I was thinking about university in grade 12, I was warned away from a gap year. It was Christmas time, the worst time in grade 12, because you are forced into rooms all those people you never see and they all ask the only thing they can think of: "What are your plans for next year?"
At one such event a university prof in the room went off about gap years being wasted time. He threw around disturbing statistics about rates of return to school post-time away. By the end a seed of doubt had been planted.
While I certainly see the problems associated with a gap year, I think they have more to do with the student than the year. If someone who is highly unmotivated takes a year to sit around playing video games and sleep on his parents' couch, that is problematic. However, if the same person attended university immediately after school, he would do many of the same things, except while paying thousands of dollars for it. A more motivated student might spend the time more wisely, considering options, saving money, and learning from his or her peers' mistakes.
Ultimately, I chose not to take a gap year, though my first year was much like a gap year in some ways. I took an interdisciplinary first-year programme. It's a self-contained year, creates the opportunity for exploration into many subjects, and allows a student to build independence. When I got to second year, I felt much more comfortable choosing a path since I had done this exploring.
I'm not sure how I would have spent a gap year. Working would be valuable, and travel could be enriching as well. One of the most compelling reasons is to turn off the university pressure from parents and teachers. Many students would do well, or better, with more specialized training in a college environment, but are discouraged from it. While I believe a good education in any subject is never wasted, it is difficult to obtain a good education when you're disengaged from it.