I love FastMail's interface and used it for couple of months. Recently, I let my free trial expire and allowed my account to be deleted. Why? Because it's not just about email. Everything, I mean everything, is in my Google account...docs in Drive, 10s of thousands of photos in G+...and it's all integrated so well. Bailing on the email part of the Google suite just adds a layer of disconnect and complexity...it's unfortunately just not worth it to me at this point.
