Also a Fastmail user with my own domain. Reliability has been excellent, their web-app is great, and their documentation is very helpful. I've never had to contact support (in over five years), so can't comment on that. Two minor complaints: (1) their spam detection is good, but not Gmail good; and (2) their file storage has a 50MB per-file limit, even though the total cap is 10GB. I have a small personal site hosted with them. I'd like to zip all my personal files and store them with Fastmail, but I can't because the file size limit is too low.