There was a great post recently at The Business of Software blog entitled Why you should burn your business plan. The gist of the article is that it's not so much what is in your business plan, it's that sitting down to actually think out, draft, edit, and forecast is by itself a crucial exercise. I couldn't agree more.
Writing in general is a painful exercise for me. I don't particularly enjoy it, I make basic grammar errors all the time, and would prefer to just manage by feel. However, anytime I'm faced with any type of significant decision - whether it's a new product, a new direction, a new management structure - I always try to get my thoughts on paper (not real paper of course). It's not particularly fun but there's just something about writing it down that makes your brain think differently.
I spent a lot of time on the original business plan for Billtrust. We were going to provide an outsourced billing solution for the QuickBooks marketplace. Intuit had recently decided to become more open with a software developers kit and it seemed like perfect timing to provide a value added business service like outsourced billing. That first business plan was so wrong it's comical. However, that doesn't mean it was a waste of time. I had set some milestones in the plan and within six months it became pretty obvious that the plan wasn't working and it was time to "re-plan" (re-plan is secret code for you better come up with something else). Fortunately for us our solution was fine, it was just that we were targeting the wrong type of customer and were able to recover.
Sometimes the writing is just for me. Other times I circulate to my management team because I don't feel strongly about my conclusions or need additional input. If I'm going to distribute externally, I always have my Mom do some editing for me.
No matter how much you dislike the process, spend the time and write it down. No one ever says "Darn, what a waste of time collecting my thoughts and putting them on paper."