I've said it before and I'll say it again, unlimited days off (or "open") and zero days off are identical. If you have unlimited vacation people are inclined to take less and people respect the vacation you take less ("as you can always take more!"). Here's a study[0] (PDF) called "Overwhelmed America: Why Don’t We Use Our Earned Leave?" It is biased (travel association) but interesting nonetheless. According to this…
More than the sheer # of days for vacation, having the flexibility to take a day off to hang with the kids or make a spontaneous trip to Iceland (happened for some coworkers) is pretty awesome.
I actually just went through my calendar and counted about 31 vacation days. I'm surprised I took so many but never did I feel like I was shorting my team in any way and felt any guilt for it. This includes a 10 day trip to Europe and basically all of Christmas -> NYE off.
Most seem very skeptical and cynical of unlimited vacation, but it's more about the culture than the policy itself.