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…
At Trulia we have an unlimited vacation policy and it really does work for us. People routinely take extended, foreign trips of two or more weeks. This year I will be taking 4 weeks off in total, 2 on proper vacation and one week each with my family and my wife's. So take this as an anecdotal data point. Oh, and come work for Trulia, we need great iOS and Android engineers desperately.
So...basically a standard amount of vacation?
I love the sound of unlimited vacation as a perk, but I'm worried I'd just get fired for "abusing it". I'd probably take something like 10 weeks/year