I'm working on my next book and I can't imagine starting a book without an outline.
It would be so easy to veer off course and lose sight of the storyline! Plus I would forget so many critical details and plot points without an outline. Character inconsistencies show up in an outline, too. It would just be one jumbled mess!
And I HAD to outline for my series. The Circle of Friends has so many characters that appear in several of the books, I had a master outline detailing the major events in their lives each year, from the first book all the way to the last book. (That was a span of over eight years, too.) It was the only way I could keep all of the details straight and remain consistent from book to book.
Needless to say, I highly recommend that everyone starts with an outline!