We have what we call "Recharge", so unlimited time off (I was up to 5 weeks of paid vacation before they switched). We switched over a few years ago. I think it's great in theory (and I don't know of anyone that has abused the policy), I find that I have to be more aware of how many days I've taken, because I won't take enough. I don't take sick days (I work from home, so even if I'm not feeling great I can still work in jammies on my couch), so I will sometimes go a few months without taking any time off. I definitely don't take 5 weeks worth, though I should.
I think it's a benefit for companies because they don't have to pay out any remaining PTO to people, and they know that employees may tend to take less time than what they would've earned.
ETA: I'm fortunate in that our company encourages us to take time off. It's a personal thing that I don't take more time - my work load falls to others when I'm out, and although I have a fantastic boss and team, I feel guilty! There are definitely times when those in our role (we are in sales) don't take off (end of quarter, end of year). But not because that's forced.