I'm thinking about migrating (gmail just feels sluggish), but I'm a little afraid of all the potential gotchas vs my current gmail setup, which makes me reluctant.
(I redirect + pop fetch several school addresses to gmail, plus have a long list of filters to help me navigate all the ads I get)
Also fastmail is faster! You get email, contacts and file storage. If you are on the $40/yr plan you get caldav support as well. Works very well on all my devices.
Only downside is spamassassin isn't as effective as gmail's spam filters, but I no longer have to cope with mailing lists being marked as spam by the masses who can't work an unsubscribe button.
On that note, you can also write your own seive rules for filtering, and there's the 'if matched stop running filters' option that is so glaringly absent from gmail.
So I'm a happy customer.
I switched because I recently purchased several domains and needed a mail system for them. Google Apps isn't free anymore, and the free email forwarding from my registrar just expired.
I will still keep my personal emails on Gmail for now, mostly because I love Mailbox.app too much.