Absolutely, very happy Fastmail customer here! My only qualm is that their apps could be better at multi account (switching is too much of a hassle), but that's the only problem I ever had, and I work around it by using different mail/calendar clients.

If you just have a single account with them though, their app is quite excellent and has everything (mail, calendar, notes), no need to get multiple apps and stuff like DAVx unless you want to.