I still use mutt when I really want an email powertool, but I can't use it as my daily email client any more (and haven't for years) now that HTML emails are so prevalent.
I use lots of plugins with Thunderbird (Copy Sent to Currrent, Enigmail, External Editor, Nostalgy, QuickFolders, Identity Chooser, Mail Redirect, ...) to try to bring back some of the functionality I'm used to with mutt and it works quite well now.
In recent months I find Thunderbird needs restarting once a day which is frustrating. It goes into some kind of internal loop processing an email and never returns. Probably a consequence of too many plugins!