The best feature of zen shifts is that you can see who's working where at any given time! So if I'm off site for an event or training they know to cover my shift while i am away so there are no issues with covering someone else's vacancy in their absence.The only problem we've had was during our annual summer shutdown when there were some vacancies left unfilled which meant people didn't get paid until after Christmas as per pay run rules but this could be solved by ensuring everyone gets paid at least once before christmas.
Zen Shifts has made managing rostered employees much easier than other software systems previously used (and still being used currently).