I have used free busy for about 2 years now as my main calendar tool of choice in addition other tools such like google hangouts etc..
The interface isn't great but you get use too it after some time! It's very easy tp create new events (if not already there) or add existing ones if they are available through another application / method that i'm currently working from/using atm so no issues here really - just make sure any calendars exist first before adding them otherwise things may be added manually which takes longer than necessary sometimes :). Having an alternative option would definitely help people who don’t want Google Calendar itself installed eithout having problems managing their schedule effectively when something goes wrong wth one system eg lost password ect.