I switched from Gmail to Fastmail about a year ago after using exclusively Gmail for email since 2007. I've been very happy with Fastmail since then. Primarily I was bothered by getting the service for "free" from Google--it's not free for them to provide it (no such thing as a free lunch), which means they have to find ways to monetize my personal communications with others without me directly paying them. Obviously there are the ads, but then there's the matter of having to stay up to date on their privacy policies... long story short, Google creeps me out a little, and I'd rather pay a bit of money to feel like I'm not a host for some kind of sprawling global parasite.

Anyway, pros:

- The Fastmail web interface is fast. I've never seen it take longer to load than 1 second on any computer or internet connection; it's noticeably faster to load up the entire interface than Gmail.

- Fastmail also uses basically the same keyboard shortcuts as Gmail (j/k to go up and down, x to select, # to delete, y to archive, z to undo, r to reply, f to forward, a to reply-all, ctrl-enter to send--works from any field in fastmail rather than just the body or title as in gmail, which is handy for forwarding things).

- Sometimes sending/opening messages in Gmail was kind of slow; sending messages from Fastmail has always been virtually instantaneous, and opening them is definitely instantaneous. I think Fastmail preloads email content into the client so that it only has to request images from the server, because it definitely opens email too quickly to have done a round trip to the server to get the email contents. If you press 'o' to open an email, it will be open by the time your 'o' key comes back up.

- Fastmail can help with setting up DNS for emails for custom domains.

- Fastmail has this "Pin" feature that lets you pin emails so that they stick to the top of whatever folder they're in. I always wanted something like this in Gmail (like if starred messages stuck to the top or something). I think there's an option to toggle whether or not they float to the top if you're not a fan. Anyway, I'm not sure that having this feature has actually been beneficial to me in any way, but I like it a lot.

Cons:

- The Fastmail interface is not as pretty as the Gmail interface. There is no option to get a big, beautiful background picture, and the interface doesn't really have space for it anyway. This bothered me a little at first, but these days I don't care... if I'm gazing longingly at the mountains in the background picture of my email, I'm being extraordinarily unproductive.

- Fastmail's auto-complete when you're typing in the recipient list does not keep track of who you send to most frequently and push them automatically to the top of the list. This still bothers me sometimes.

- Fastmail's settings pages are a little confusing. There are a couple different ways to get to them, and it can be a little tricky to figure out where you're supposed to find something. I've never been a fan of the way settings are organized in any email client, though... Gmail and Outlook have settings pages that aren't as terrible these days; Fastmail's settings are a little less refined. On the other hand, it seems like you can do a lot more with the settings in Fastmail (so this is a bit of a pro, not strictly a con).

- Fastmail has a folder system with nesting like Outlook. I don't think this is really a bad thing; I like the concept of Gmail's labels a lot, but I never found it to be significantly more useful than folders. What sucks about Fastmail's implementation is that to create folders or rearrange them, you have to enter this special folder management menu. Then to create a folder, you have to scroll all the way to the bottom and type the new name, and if you want it to be at the top, then you have to drag it up past all of the other folders, and there seems to be a max speed on how quickly you can drag it up, and it starts arbitrarily selecting the text as you're dragging and sometimes gets a little janky as you get to the top. Definitely my least favorite part of the interface.

Ambiguous:

- Not strictly a pro or a con in my opinion: there is no chat interface built-in (actually I'm not certain about that, there might be some option to enable chat and put in chat credentials, etc. but I haven't looked for it). Overall I think this has been very good for getting me in and out of my inbox very quickly and productively without any risk of distraction. I actually stopped using any form of chat until more recently when I needed it for work, and I think my quality of life was improved for it, but I know that for many people chat is an important part of communicating. But I haven't missed it in my webmail one bit.

Wow, that was a lot more than I expected to write! Anyway, to sum up, from my perspective as a web developer:

- Fastmail is faster and contributes better to getting things done, in my experience (worth the $60/yr for that reason alone)

- Fastmail has some nice options if you're setting up email inboxes for yourself for custom domains. I don't know if I would recommend it yet as a mail option for clients since it's a little less polished than Gmail and some of the settings options are a good deal more technical.

- Fastmail is a nice gateway drug to freeing yourself from the idea that Gmail is the be-all-end-all of email. Still waiting for someone more productive than myself to combine webmail, Trello, and Google Calendar into a glorious productivity app that ushers in a new era for humanity.