My buddy Healy Jones of OfficeDrop wrote a terrific post about managing your startup through a spreedsheet (and not a business plan). I couldn't agree more and it's how I evaluate businesses. I find myself prompting management teams for the numbers when they want to talk about "where things are going."
Btw - Healy is writing great posts about the entrepreneurial journey. If you are in that boat or want to take the journey someday, you should subscribe to his feed.