At my last company, I led a team to build an experimental product I skunk-worked a few months prior. While it wasn't the "90 days on whatever" freedom, I gave them the keys of responsibility for all aspects of the project. I took a back seat. I'd pipe in for a couple of high level meetings and keep other projects from infringing on their time as much as possible.

For the project, it was an unbelievable success. The technology and infrastructure built went beyond my initial vision and the developers took to the project with gusto. Moral was high and development breezed by.

Unfortunately I couldn't stop management from bombarding us with fly-by-night projects. I say that literally. They came unexpectedly and left just as quickly. We operated in a perpetual 2 week fog of war; there were no plans/discussions over these projects (including goals) and completed projects were abandoned almost as soon as they were done. When these projects overwhelmed the original purpose and drive of the team, moral dropped like a stone until several members resigned.