Like most things in business, the problem isn't with the policy, it's with management. If you create a culture that frowns upon employees taking vacation time, it won't matter if you give them unlimited vacation or only two weeks, nobody will take it. And if you create a culture that values rested, refreshed, happy employees, the total number of days isn't nearly as important.

With that said, when unlimited vacation time is implemented well, it can be amazing for both the company and employees for a number of reasons:

1) You don't worry as much about things like sick days or one-off PTO. Work doesn't always follow a 9-5 schedule, so if you end up working 60 hours in four days, you can just take Friday off - it feels like less of a big deal if you don't have to worry about 'losing a day.'

2) Do you need to leave early to make a flight? Don't worry about entering it into the system, just do it.

3) If people are legitimately sick, they won't come to the office and infect everyone out of fear of 'using a sick day.'

4) Holidays get a lot easier. Do you not mind working between Christmas and New Years, but want to take time off for Eid/Rosh Hashanah/Diwali/Festivus/whatever? All of a sudden it's not a big deal. It's also great for parents, who don't have to worry as much about child care if their kids have random day off of school.

5) You can take a 'big trip' without losing all your days. The consecutive days, rather than the one-off long weekends, are the bigger issue for a company. I've actually found it easier to take a long trip when I didn't have to worry about losing all my days.

Companies are nervous about this arrangement because they worry employees will abuse it, but again, that's a management issue. You have unlimited vacation time as long as you are getting your work done. If you're taking so much time the work isn't getting done, then the issue isn't vacation time, it's a performance problem, the same as if you showed up every day and just played on facebook.

Because the management piece of it is so important, there are also things companies can do to make a system like this work better:

1) Create a mandatory 'real vacation' by requiring employees to take at least a week off, in a row, once a year. If employees feel like they can never take time off, they'll get burnt out and you'll lose great people. If you make a vacation mandatory, people will feel more comfortable taking the time, and the company will reap the rewards of happier, clearer thinking people. Plus, many people, especially in creative or professional jobs, get inspired when they're able to step away from the day to day for a little bit.

2) Make telecommuting easy, so that employees can still check in even if they're not in the office. Sometimes, people only really need a half day, but don't mind sitting on e-mail in the morning. Or they have their kids around all day and would love to get the opportunity to talk to some adults. The important thing here is to be flexible, and make sure that telecommuting doesn't become "we can make you work wherever, whenever, even on vacation."

3) Have real performance management. This isn't about annual reviews, it's about managers that provide ongoing, regular feedback about how employees are doing against the company's goals, help them understand when they're falling short, and work non-performers out of the organization. Top performers will get turned off by anyone abusing the policy, even more than management.