This might be the feature that makes me switch my personal email.
I somehow (not objectiviley) like the impression Fastmail gives me. The cowboy-ish yet serious way they try to compete with the big ones. That might be a reason to join too.
They also have stellar support.
Also the features are different, Google has some FM lacks vice versa. Depending on the features you need that might be an advantage or a disadvantage. Personally I only miss "Send & Archive" from Google.