[erlehmann_]

I use both the Gmail web interface (at work) and notmuch-mode from within Emacs as an offline client. My evaluation: notmuch has a nice local search interface, supports tags and threaded views and handles PGP, but is similar enough to the Gmail interface. I find it much easier to use: https://notmuchmail.org/screenshots/

By coincidence, I used Sylpheed before and Thunderbird before that. I found both easier to learn than notmuch-mode, but ultimately lacking, especially regarding handling of encrypted messages.