I am using https://www.fastmail.com with great results.
They do have some features in there interface to ease the transition like being able to both send and receive mail from your old address directly from there interface.
CalDAV access to my calendar has been very stable and CardDAV access to contacts (the feature I've been waiting for) is in beta now. I have seen a small bug related to shared address books (I have a "business" account) on Yosemite, but other than that it has been rock solid.
Some nice things they have: * Proper IMAP and SIEVE support. * CalDAV, CardDAV. * The UI is keyboard-only usable. * The support team is really responsive (they actually added a shortcut for an option that was lacking one within a few days of my request!). * They give back a lot to the FLOSS community, which I really value (but their webmail is mostly proprietary).
Their usage of actually standards (instead of just their own) is a great thing! I still use mutt for gpg email, and khal for calendaring, which is pretty good.
They also host your XMPP server, if you're an XMPP user.
I've been meaning to post a thorough review, but haven't had the time to polish it. I'll post it in HN once I have it, since I'm sure many will be interested.