There were worries initially by some about employees abusing this policy, but that has not happened once in the 6+ years we've been around. However as we've grown, I strongly suspect that the opposite is happening, i.e. employees are not taking as much vacation as (I feel) they should, perhaps due to the feelings of guilt mentioned in the article. This sucks because, unlike jobs where vacation days are tracked, we can't even compensate them for any unused days.
So over time I've come to believe that unlimited vacations days is something that sounds good in theory but not so much in practice, especially as a company scales. I would instead recommend the traditional model of tracking vacation days, but with a very liberal number of days (vs. the typical measly 10 days a year or whatever).
Case in point: our office has been closed for the holidays for the past couple weeks, and I haven't seen evidence of anybody doing any serious work on non-side projects (outside of the unavoidable on call responsibilities). Nobody had to use precious PTO for this or feel bad about traveling overseas to visit relatives.
I'd say it's highly dependent on company culture, though. We were always encouraged by our founders and office managers to take plenty of time off, and they led by example in this way. If that wasn't the case, it might not be working out well for us. I've certainly heard of other companies where it leads to less vacation.