Obviously this is just anecdotal, but I work for a company with an unlimited vacation policy, and I've actually enjoyed it. I probably average 4 weeks of vacation per year, which is what I had in my previous (non-unlimited) job. But, now if I want to take more vacation one year and less vacation another year, I don't have to worry about whether my plans fit into whatever arbitrary limitations my employer might impose.

I've also never had a vacation request denied, and nobody has ever given me any crap about taking too much time off, although not everyone does take as much vacation as I do.

I'm sure there are companies and situations where "unlimited vacation" does end up translating to "no vacation", but I feel like a lot of the complaints about unlimited policies stem from people who are overly insecure about their careers. If you do good work and are a valuable asset to your employer, nobody is realistically going to fire you for taking an extra week off now and then.