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I've been on Fastmail the same amount of time. tl;dr: I encourage you to switch.

One of the be things about fastmail is they've been rolling out noticeable improvements to their web apps over the last 2 years.

I'd say the downsides, at this point, are the labels model for emails is slightly different than gmail. If you've decided you really like how labels work, Fastmail's don't work quite the same. And their contacts app isn't good. Though given the pace of improvements elsewhere, I suspect they're working on a fully-featured contacts app.