We have an unlimited vacation policy at Adzerk and I try to lead by example by taking at least 2-3 weeks off a year (especially in the summer). We also try to encourage everyone to take at least one solid week off a year and it seems to work pretty well. I think that's the real issue with unlimited policy - if the leaders don't take vacation no one else thinks they can.
Is 2-3 weeks off a normal amount of time in the US? I get 28 days off per year in the UK, which is around the standard amount.
Some companies are more enlightened than others, however.