While I still recommend the business planning process, I caution you to realize that a beautifully crafted document does not always equal business success. I've worked with many entrepreneurs who successfully launched without a plan, and some with beautifully written plans that never materialized. You and your business idea are unique. Your planning process will be unique as well. Be wary of one_size_fits_all advice or pronouncements from experts about how you should proceed.
Then there are the moments when something seems to be going wrong, when one or more areas of the business just don't seem to be working. Cash flow is anemic or the marketing message is flat. Perhaps customers have shown a marked interest in only one particular product or service, ignoring all your other offerings. This means it's time to revisit your business plan, more precisely it's time to revisit the questioning process that helped you craft your plan.