What I've found is that those days of pre-planning can be hit or miss. Yes, you might uncover a design flaw or missed requirement that would have set you off in

What I've found is that those days of pre-planning can be hit or miss. Yes, you might uncover a design flaw or missed requirement that would have set you off in the wrong direction. However, a prototype implementation is much more likely to uncover those same flaws, and probably also a few that you never would have found without building a prototype. I've found the prototype approach leads to a far higher likelihood of success. I think the real challenge with it is getting management to understand what is a prototype and what is a finished product, the danger being that you ship the prototype. Of course, this is all highly dependent of the nature of your work and length of development cycles. But if you're shipping in weeks, I'd wager you should be planning for hours at a time, not days.