After round about 15 years of using Mutt (most of the time, it simply sucks less), Thunderbird (several interludes, the longest one lasting about 3 months) and K-Mail (never longer than two or three days, never got warm with that) I now switched my email reading and writing to Emacs/Wanderlust.
In my opinion most of the modern email clients are broken. They don't support real threading (see for example Apple Mail's conversation view — WTF, mail clients from the 80ties have been able to display mail threads properly), are terribly slow or are not convenient (for example, which mail client can be used with only the keyboard?). This is why I've been using Mutt the whole time.
I've never been a friend of the „do everything in Emacs“ front, but recently I've been reading a lot about why using Emacs for email is great. One of them is a tight integration into org-mode — org-mode is one of my most-often used Emacs packages. I use it for nearly everything regarding notes or time or time tracking in my live: todo tracking, project planning and tracking, tracking appointments, etc, pp. Simply everything. So a tight and good integration would be a big deal for me.
So I decided to give it a try. First I tried to use Gnus since this seems to be the first choice for most of the Emacs users. But I want to use isync/mbsync: this setup has proved its advantages so many times that I don't want to switch. The problem is, that Gnus has a terrible maildir implementation. Thus I decided to give the other choice when using Emacs for email a try: Wanderlust.
The first impression was pretty weird: setup was rather complicated,
even more complicated than Mutt when I've been using it the first
time. I had to set a load of variables and write some
defuns until it worked like I'd like it to work. It is
poorly documented in the setup and customization phase, you
basically have to search the web for weblog entries and tutorials
and/or read the souce code. On the other hand you can customize
nearly everything, my current
currently contains 236 lines. So you have to be enduring, but in the
end it pays off.
The org-mode integration, on the other hand, is really great. I'm able to create entries attaching a link which I only have to visit to get a mime-view buffer with the email. Yay! Additionally you've got all the Emacs power when it gets to editing text — which isn't a real advantage since I've been using Emacs to compose mails with Mutt for ages.
But all in all it feels like the right decision to switch to Emacs for mails. I'm curious how it will work out in the future!