You can talk IMAP/POP and SMTP, and you get to control your own user interface, or you can talk HTTP, HTML, and JS and abdicate control to someone else. I fell into that trap myself. I was a heavy user of Gmail for about four years, and when they started changing things and told me "only 0.07% of users use that feature we just removed", it was too late.
This mayhem ruined "the cloud" for me ... and I used to help run parts of it. Blah.
For GUI clients, Thunderbird is the best I've seen, followed by Evolution (back when I used it, anyway).
Though if you're open to a ncurses-based client, I use mutt. The #1 reason I switched to it (instead of any other client) when the new Gmail design was released was because it opens almost instantaneously. It's a subtle thing, but not having to leave my inbox open all the time and being able to open it in a split second when I need to really helps my workflow.
This mayhem ruined "the cloud" for me ... and I used to help run parts of it. Blah.