Great post! I'll paraphrase Scott Rafer: "An entrepreneurs success should be measured by the cash he puts in his pockets." By that measure, I think Dropbox is currently a success-- Drew is (or should be) making more money than he could if he'd gotten a job instead of founding Dropbox. For Drew, it'll be MORE successful when it exits. But if it crashed/burned now, it'd still be a success for him.

Of course, every startup has a different objective-- and every stakeholder has an objective.