Had some initial buyer's regret for no reason in particular (did the 3-year plan for the best price and assuming prices will go up over time), but already feeling really solid. The web client is very snappy, documentation is good, able to import my Gmail messages, contacts, calendars, and even filters in a snap (though imperfectly). Looking forward to seeing how the API compares with my Google API scripts. Definitely seems to be more knobs and buttons for power users (more powerful filters, regex, etc.).
While not relevant to FastMail per se, also really glad to finally start the painful process of migrating decades of accounts to my custom domain email, where I hope it will stay forever. Growing concern about the (admittedly slim) possibility of losing access to my Gmail account was a big motivator, as was the recent move to force the Google Apps stuff to a paid plan.
So far so good.