That is definitely the perk that isn't. Imagine a startup offering employees unlimited pay -- "Whoa whoa whoa, you drew $3,000 this month? $3,000!? You're a single guy, what do you need $3,000 for? Don't we give you enough soda?"
My employer has the same policy, and I've enjoyed a ton of vacation time (in exchange for getting a lot done when I'm not on vacation). Unlimited vacation is an awesome perk in the hands of the right employer.
It really is dependent upon your work environment, and the people you work for. When I started at my current job, we had two weeks vacation after 6 months, plus half a dozen holidays. We were a very young company, and I got the impression that policy was created somewhat arbitrarily when we only had a couple of full time employees.
We went to an 'unlimited' policy about 16 months ago, and one of our devs spent almost an entire month overseas earlier this year without issue. He's not too much of an outlier within our company in terms of how much vacation time is taken, either.
