We can't stack too many days up because you can only carry over 40 hrs into the next year.
Forces you to "use it or lose it." We also get 2 or 3 days for health and wellness every year.
It's tricky because I want to do fancy travel early in the year when it's cheap, but don't have the PTO saved up. We end up swamped at work from 4th of July until mid-November, so it's hard to take off in summer or early fall.
Get to the end of the year and I always take the week off for Thanksgiving (only 3 days since we get Thurs and Fri off by default). Then I'm left with a shytload of PTO in December, so I end up taking like half the month off.