I use it daily. It's a lovely email client. Close to the Unix ideal of doing one thing, and doing it well. Integration with other utilities (notmuch, isync) for indexing and sync'ing is very easy, thanks to close adherence to Unix principles, and gets you a very neat setup. Nothing to envy from Gmail.
I personally always felt that "full applications" were disconnected from the Unix philosophy. By definition, such an application does many things: fetch things from the network, display them, sort them, ... it talks many protocols, concerns itself with many I/O systems. I'm certainly not saying that it is the way to go, but a real Unix philosophy way of dealing with email would be to use a command to pull mails from…
That is, it just reads emails on a local mailfolder. Fetching emails to that mailfolder from the server, sending emails and doing other operations is a task of 3rd party programs.
[1] OK, it has a few addons for lazy people. But most experienced users ignore them.