Ask HN: Does your company offer unlmited vacation?

I'm wondering for those of you working at companies that offer unlimited vacation days, how many do you actually take in a year? I'm seeing more companies tout unlimited vacation as a perk of working there (e.g. Listia, Automattic), but how many days can you actually take? Are there blackout days around releases? How does this work with project management (e.g. in Agile, are you excluded from a sprint)?