Yes! The last 3 places I worked had unlimited vacation policies. Only in my current job (the last of 3) do I actually feel really comfortable requesting time off.
Whenever I tell relatives/friends that aren't in tech about "unvacation," they immediately get excited and say how they'd "take half the year off!" Uh, no, doesn't work like that... I usually describe the difference as entitlement vs privilege.
If you get, say, 4 weeks vacation per year, then that means you are entitled to 4 weeks vacation per year. The only justification you need to take time off is that you have vacation days. With the "unvacation" policies, you feel obligated to somehow justify the time off. Sure, in many jobs you still have to be careful about your timing of your vacations, but it is seen as a failure of time management if you get to the end of your year and still have vacation left over. It is expected that you will use your vacation.
Additionally, you start looking at your coworkers and, worse, your Boss's vacations and use that as a sort of measuring post for how much time off you can take. This resorts in a race to the bottom, as everyone focuses on the employees that work the most. Imagine you want to request your 4th week of vacation for the year, but you know your boss has only taken 1 week and usually works weekends.
Case in point. At the first job I saw an unvacation policy, we made the transition because (paraphrasing here) "some people were working really hard, and the company wanted to let them take an extra day off, if they worked through a weekend or something." Imagine your boss just said this to you. That you just had your work time equivocated as 1 weekend = 1 day off work. How do you then go and request a week off?
It can work, but it all comes down to your manager and company culture. Unfortunately, most tech companies have a crappy culture, promoting the workplace (and hence work) as a replacement for a social life. And most startups have very flat structures, which means that your boss is usually a founder who works 80h weeks. Anyways, sorry for the rant.
You: "Hey Boss, I'm taking a week off."
Boss sees you have only taken 2 weeks so far.
Boss: "Great, have fun."
With an "unlimited" vacation policy there is no the added "human" factor.
You: Hey Boss, I would like to take a week off. It's been a rough few months, but we finished features X, Y and Z and we have a lull. Is that OK?