I've been leaning this way, I've also heard of people actually taking less time off with the policy. Although I'm not sure how much of an issue that will be as we are usually pushing people to unplug a bit. Also the partners do a good job of leading by example and taking time off. The fact that you don't have to track the vacation as much given everything that has to be tracked in the business is enticing though.
I think it really depends on the company and the founders: I have seen it both ways with friends. There are folks who don't even take a single day off every year in spite of having "Unlimited vacation days" because of a poisonous culture where people who take days off are considered to be slackers by peers. I think this is due to leadership failure because one of the reasons this happens is because the company is so "lean" that they can't afford to even miss one person for a single day. On the other hand, there are places where the CEO forces people to switch off their phones and GTFO because they are sick and have been working their asses off.
As an employee, I prefer the two weeks deal because it is clear and concise and there is no b.s. about whether I can take days off or not. My company does that and also shuts down during christmas holidays from the 24 till Jan 01. I feel like that in all makes a pretty decent holiday package.
Also, to answer your question regarding the fact that this is another parameter to track. You could just get people to self report PTOs on whatever HR management system you use?
Didn't know these guys were still running. It's an interesting take on a interface that we're used to on the desktop. This is an open source project to me though and not a business. Build out your interface to host on other platforms (think DISQUS) and you guys would be solid.