The ability for people in different time zones or countries, it's very easy! It can be used by my colleagues from other companies as well so we don't have any issues with that at all either (we've been using this tool since 2006).
I also like how you are able of easily adding your own reminders if needed too - which saves me some work sometimes when there were meetings where no one was around nor could attend them because they weren’t notified about those yet etc., but now everything works perfectly fine without having these situations ever again thanks :) Sometimes though i wish notifications would show up faster than what currently happens here; however nothing major really comes into mind right off hand on why not do something better regarding notification speed-up :D Overall great product/tool!! We use Zocal internally mainly only within our department & partners offices across Europe / US timeszones + Asia.