I've been with Fastmail for two years, and it was probably the best IT decision I've ever made.
http://www.fastmail.fm
The front page and mail screen look slightly odd because they are optimised for speed - the interface is so responsive that I completely changed my mind about Webmail and don't bother much with mail clients any more.
Fastmail is pure Open Source server software (Postfix, ClamAV, Spamassassin, Cyrus IMAP etc.), and they encourage users to use secure IMAP or the HTTPS Web interface. The company running it have been around for five years and have a large subscriber base of power users. They get enough business by word-of-mouth recommendation that they don't advertise.
I actually do mail server administration at work, but there is absolutely no point running my own system for private use since FM already do everything I would if I spent a lot of time setting up the whole thing myself. At a very good price, too.