I'm doing an internship this summer with a Utah company that has unlimited PTO. Because of the very healthy company culture (where executives openly talk about their family vacations and "pacing yourself" is part of the written company values), the PTO policy is a very good thing.

Like the Forbes article mentions, I see a lot of folks opportunistically taking vacations-- extending weekends, etc. I think the policy provides (or at least seems to provide) greater flexibility. If you're in the midst of a big project, take a day or two off to recharge. If you've just finished a big sales push or ended a quarter project, take a longer vacation.

I can understand the fear of a manager's arbitrary definition of "reasonable" or "excessive", but I've also seen the benefit of the system in producing much healthier and happier employees.