I work at Factual, which has an unlimited vacation policy. The amount of vacation that people take varies greatly.
At the high end, I've taken about 8 weeks of vacation during each of the last two years. I tend to do some work during vacations, however, so it's more like 8 weeks abroad in which I happened to squeeze in 4 weeks of work at odd hours. I think a few of my coworkers also take 4-6 weeks per year. Most of these people are pretty disconnected when they travel.
On the low end, some of my coworkers don't take vacations at all, or take one week per year and then feel guilty about it. I understand their guilt about as well as they understand my wanderlust =).
Things that affect how much vacation people take:
* How long you've been at the company. Someone who just joined is much less likely to take a vacation than someone who has been around for a few years.
* Your place in the company hierarchy. More senior people seem more comfortable with taking vacation than more junior people.
* Schedule pressure. If you're working on a huge project that will take 6 months, you probably won't try to take a vacation during that time.
* Being the only person on a project vs. being one of many. If you're the only person who knows how something works, you tend to worry a lot about what can happen while you're gone.
* Personal budgets and travel preferences. If you like hiking trips, 6 weeks of vacation won't set you back very much. If you like overwater bungalows in Tahiti and private cruises in the Galapagos, then you need to have a lot saved up to take more than a few weeks off every year.
* How much you like to take vacations (or how much you love your work). Some people like work enough that they don't want to take vacations. Others don't really care for vacations, so they might take a personal day or two but not go on longer trips.
* Your, ahem, cojones. Generally, if you ask your boss reasonably, you will get a "yes" 90% of the time. A lot of people are afraid to ask.
When our unlimited vacation policy was established, it seemed like people were waiting for their peers to set the boundaries of what was appropriate and what was not. Over time, I've realized that people are just different. People who don't take breaks don't seem to change their habits even when they see others take frequent vacations. Conversely, people who take frequent vacations don't seem to tone things down in the presence of non-vacationers.
If needed I take my laptop and put in any required work remotely - that's great when you work on web tech. I do this when I know my days aren't going to be busy, such as visiting family. If I know I won't have time or access to the net on a trip I let everyone know so they can expect I will be disconnected.
I've taken about 4+ weeks off so far this year, 2 of which I was working remotely, and will be going to Burning Man this year for a week. I'll probably take some time off in December.
The key is to make sure you are being productive and adding value when you are at work so you don't feel guilty taking time off. When you plan off time, make sure there isn't anything crucial to the business happening at that time. Let people know how you are going to be accessible (IM, email, ability to commit code) or if you are off the reservation.
There is a certain amount of uncertainty and self-inflicted guilt about the whole thing; whether you are taking less/more time than your peers. I guess that's the price you pay for this perk.
There are any number of ways it could be abused, of course. But the thing to remember is that your coworkers (and boss) aren't blind - if you're using the policy to create a 4-day workweek, and not getting stuff done -- someone is going to say something.
At OKL, i suspect the policy is just an extension of their "ok, we are all adults here, so just act in the company's best interest at all times--we don't want to work in a soviet tractor factory, we assume you don't either" (my own words, if we had an employee handbook, i imagine it would say something like that).
at other shops, i have heard that the motivation for unlimited vacation is a little less wholesome. The idea is that use it to recruit, then once in the door, the manager can of course limit the employee's holiday because his approval is required. So there's very little to lose, and a lot to gain; when the employee is terminated (resigns, fired) the company does not have to pay out their unused vacation, which is often not a small amount (eg, i suspect a lot of people carry around 2 weeks).
However, in a healthy company culture which encourages employees to take time off, it can be great. Particularly because long trips are really a different beast than short ones, and it's hard to do a long trip with 15 vacation days. Solution: take 7 vacation days one year, and 30 the next.
The biggest thing has been to make sure that you aren't leaving anyone hanging. This is probably a bigger deal for some departments/teams.
One company I know has introduced a bonus for taking a set minimum number of days to try and make sure that their employees are taking a decent amount of holiday each year. So for example if you take 20+ days off in the year you will get a annual bonus of £1000.
First you ask yourself if you have critical work to do those days. If not, you're good. Otherwise, you may have to get it done sooner. Then you ask your coworker if they're going to be around to cover for you. If they are, cool, you've just scheduled yourself out of office. Set up your OOO notification and update your calendar. If not, check with your manager to make sure it's ok for both of you to be out. Generally this may mean a concession that you have to carry your laptop or phone in case stuff goes down. Congratulations, you've now scheduled your vacation.
The only time our manager really gets involved is during Nov/Dec for the holidays when people run into "use it or lose it", so he'll informally let us carry over to January or so, just so that everyone isn't rushing to take at the same time and leaving us without anyone in the office. The other time is when you want to take multiweek vacations, just so he's aware and can get coverage information.
Even then, half the company is in the same situation so it's normally pretty dead in Dec.
I no longer work there. :) And in fact, I since turned down an offer at an otherwise appealing company specifically because they told me they had an unlimited vacation plan. I took the job at a company with a specified number of days, knowing that I would actually be able to use them and not feel pressured to never take time off.