I've been using them for 10+ years, with no complaints. Their webmail is good, never any problems with their IMAP, a bunch of domains to choose from for their free accounts, and relatively cheap paid plans with decent quotas. Plus, they don't read my mail to sell me ads like Gmail does.
Same experience here, although I'm a newer user who just switched from Gmail a few months ago. Spent a couple of weeks researching alternatives before settling on Fastmail and would not be happy to repeat the process.
