Very happy Fastmail customer here. Find the web interface more responsive than gmail and the IMAP implementation seems to be play better with apple's mail clients.
The mobile app is also fantastic, but I've had instances in the past where I've received no notifications if an unread email is left in my inbox. I'm the kind of person that won't check my email unless I get a notification, so there have been times when a week has passed, and I've gone to wonder why I'm so unpopular, only to find a hundred unread emails from people wondering where I am.