I am a Gmail user. I personally find the Fastmail interface a lot more performant; it's very responsive for me in a way that Gmail just isn't, though I'm not a Gmail 'power' user, so my experiences might be different (and I don't have 10,000s of emails in Fastmail yet). However, I can say I've found Google's Inbox product really useful a number of times recently (automatically classifying emails as travel, purchases, etc.) and I'll definitely miss that.