I've been putting off switching away from google apps for my personal domain until fastmail had the tripod of mail+contacts+calendar in place, now to wait a couple of months for the initial bugs to be ironed out and it's happening.
I've ben using beta.fastmail.fm since I signed up. The calendar has been in Beta for months. Big bugs would most likely already have been fixed.
Always there is a difference between "bugs discovered in beta" and "bugs discovered when you let your application loose on the wider public". I don't have time to deal with bugs in my calendars, so will wait.