My first "real" job in software was at a startup with unlimited PTO. In my experience what this meant was that the rank-and-file engineers took no time off while management would be out of the office for months at a time.
same experience. the only person who used it in earnest was the head of HR. For lower tier employees it was much worse than a given X number of days - they had to request the time off from their manager and could (and were) denied taking time off.