I'm happy with it. At my last job I felt miserable negotiating paid time off past my limit:
"I'd like to take a week off."
"Well you only have 4 days left, and we need at least one person in the office the day before Christmas, so I could turn this down..."
"But I'm only short by one day!"
(I end up delaying my Christmas vacation by 3 days as a result, and I resent my boss for it)
This doesn't happen at Knewton. If I need time off, and my work is done, I take it.
Hurricane Sandy is actually a perfect example. I've had no power since Monday night in Soho, and today is my first full day online. An unlimited vacation policy means I don't have to take a shorter Christmas break because an act of God swept through and pwnd my neighborhood.