I've always liked Mail.app, but in the past year or two it has become slower than I'd like and it doesn't work with GMail as well as I'd like. So I've been on the lookout for another email application when last summer I saw MailMate. I tried it a bit and although it was a work-in-progress, by September I had purchased a license. It was shortly after the first of this year that I finally went to using MailMate full-time and leaving Mail.app closed, but I wish I had made the switch earlier. The email applications that I've been using all these years, have essentially the same paradigm. Now that I've been using MailMate full time I found that I've been moving away from that paradigm to a more efficient way of viewing, writing, and navigating my email. Every now and again I'll discover something new and useful about MailMate and become yet more efficient in how I do things. Some day when I have a better bird's-eye-view- of MailMate, I'll come back and write a better review. For now, I'll just touch on a few items that have really made a big difference with me.
1) Layout.
This is under the View menu and lists various layouts for your main viewing window. You can even customize them to make your own, but I haven't done that. My favorites at present are "Correspondence" and "Thread Arcs". Correspondence adds an additional small pane which lists all emails (not just those in that 'conversation') between you and the person whose email you are currently reading; it is unobtrusive, but really convenient. Thread Arcs indicates which emails in a 'conversation' have been sent as a reply to which of the other emails. Very handy sometimes.
2) (nearly) Automatic Search.
As you're reading an email, suppose you want to find others with from the same sender (or to the same recipient). Just double-click on their name and instantly a search will be performed on that and you'll be shown all those emails. Now suppose you want to see all those with that subject; just double click on the subject and a sub-search will be done. Astoundingly useful.
3) Fully Logical Searching.
You can search on any item (in the header, subject, body, email client, anything else imaginable) using ANDs and ORs in any combination. Incredible. Full flexibility! At least I have not been encumbered by it yet. From the keyboard you can navigate between all the buttons and fields to search on 'Subject' either 'Contains', 'is', etc, as well as choosing ANDs/ORs. This alone nearly makes labels and sub-mailboxes unnecessary. These searches can be saved as Smart Mailboxes.
4) Smart Mailboxes.
Much smarter because of the flexibility of the search.
5) Edit as new message.
Permits you to take a previously written email, and get a 'New Message" window set up with that email in it. It is all ready for you to edit/adapt how you want and send it on.

The author (who is a responsive as they come) adds thoughtful features from a fresh view and does them well. Then he moves on to another feature and gives it a fresh look. So MailMate might not have all of your favorite features yet. Some of those will be coming over time, and some you might find will be replaced by a better (but different and related) feature he'll add.

At first I wanted to hold out and see how MailMate developed, but
(a) it works much more nicely with GMail than Apple Mail (regarding Deletes, particularly);
and
(b) even though (as the author admits) there are a number of features he has yet to add, there are still those surprises so typical of the Macintosh (where you try something that would seem natural and it works just like you'd expect).

So I couldn't hold out any longer. I also admit a bit of a selfish motive: I wanted to support its future.

I haven't tried this, but it would be nice to have one for the iPhone...to enter the stats onsite while they happened!