I've been using Emacs for email for a couple of years now. Not for everything, for now my personal email is still Gmail but work is sent and read through Emacs. I'm currently using mu4e and while it has a few quirks and limitations, I like it very much. You can take a look on how I configure and use it here.
It's pretty cool. You can invoke it whenever you're in the headers
view you can just run
M-x mu4e-conversation and it combines all the
email messages in the current thread into a single buffer.
It's pretty nice.
You can navigate between messages in the thread with
C-c C-n and
C-c C-p and use most if not all of the usual mu4e goodness but on
top of that you can also toggle what the author calls tree mode by
V. Tree mode turns the thread into an org-mode tree - also
On top of this, you can also make mu4e your default viewing mode by
setting a single variable with
(setq mu4e-view-func 'mu4e-conversation).
Although it's very cool, I'm not sure if mu4e-conversation will find its way into my workflow or not and probably won't know for a while. This is mostly due to the fact that it's summer so work email slows down and I won't have too many long threads to play with this on.
I'll report back once I give it a fair shake but in the meantime if anyone out there has had the time to play with it, let us know what you think.