We have an unlimited policy. People seem to take 3-4 weeks on avg by my guess, although I'd love to see hard numbers on that as I suspect it is higher, however we don't track it.
There have been situations where it was abused, and those people tend to not last long. In general if your work is getting done well, nobody cares. If you start dropping the ball it becomes a discussion around performance expectations.
As a type A person, it has been hard for me to learn that it is OK, and in fact important, for me to take time off.
What has worked better for me is to use it more as a gradual thing. I might take a day off or work a short day a few times a month. It helps me fit life in, get more rest, and manage stress better because I'm releasing that pressure more frequently than if I waited several months to take a week off.
I still might take longer stretches off, but don't feel any where approaching what I can only describe as desperation for a break I felt at previous jobs.
As a result I'm sure my productivity is higher, and I'm much less interested in fielding the frequent recruiter pitches I get because most of the time I know the companies they are pitching could never offer me something comparable.
If you find a company that implements this policy well, it quickly becomes one of the most valuable parts of your comp package. At a certain point in life the value you place on your free time increases dramatically. So a benefit like this grows in value at the rate for which the value you place on your time increases.