I worked at a company that had "hack weeks" - most people loved them, and they produced good prototypes. But personally, I just hated hack weeks. But then again, I hated trying to work in that office at all, and trying to work on something "fun" there just felt worse than trying to do my regular job.

I think, though, if I was running a company, I wouldn't have hack weeks or 20% time - I'd try, for as long as possible, to give people extreme latitude to work on whatever they think might be useful/fun/interesting for the company - more like the Valve model, I guess.

A whole week? That's unheard of. Sounds good, sounds like my kind of down time, but I doubt most companies would embrace that time span.

There's another way to do whatever you want at work, and that's covertly. During the times you're meant to be doing other things. Depends where your desk is I suppose, or if working remotely.