I worked at a YC startup with unlimited time off. They worked us like dogs, but even they honored the "unlimited" part of that clause. Some people took many weeks of vacation a year without any punishment or judgment I could identify.
In fact, I've heard about "unlimited vacation" being a trap at companies, but have never seen that actually happen at any of the companies I've worked at which had unlimited PTO as a benefit (n=2, mind you).
On the other hand, I find that I myself do not take much vacation unless there is a "PTO economy" I am trying to game. If you tell me I have unlimited vacation days, I will take maybe 2 weeks a year off, mostly around the holidays. Knowing that I could take more time off seems to be enough to keep me from burning out.
On the other hand, if you tell me I have 4 weeks a year off, and I lose any unused days at the end of the year, I will take exactly 4 weeks every year off, like clockwork.
So, for me, I think that having a set number of days a year is better over all, as it paradoxically encourages me to take more vacation. /anecdotal.