I've been trying FastMail for the past two months or so and have been very impressed; it's actually better than GMail in many ways. Most importantly to me, it seems to be quite a bit faster than GMail for me.

One problem I've had is with their spam filtering. I've marked everything spammy as Spam (some 800 messages so far), but I can't figure out how to get things marked as non-spam (so that the personal database gets enabled). I've setup auto-non-spam folders and a lot of messages get routed into those, but they're not being counted as non-spam. I've also moved a few messages manually, and they also don't get counted.