MailMate received a 4 out of 5 star rating in the French magazine VVMac (Vous et Votre Mac). The review is part of a large special feature about emailing on OS X and it is, of course, written in French. It is possible to skim an (unreadable) online version of the magazine here (using Flash), but I have also been allowed to provide a readable PDF of the page with the MailMate review. VVMac is one of two printed French magazines on the subject of OS X. It is distributed in France, Belgium and Switzerland.
Speaking of reviews, MailMate has also received a few user reviews in the Mac App Store (in the American and Australian stores). At the time of writing this, they are all positive 5 star reviews except for a single 1 star review. The exception is a good example of why Apple should provide some way to contact unhappy app store buyers (it could work anonymously). I don’t mind negative reviews, but in this case, a buyer has an “out-of-the-box” problem and I cannot do anything to help. Others have written wise words on this subject.
You are very welcome to provide a review/rating in the Mac App Store, but if you have some specific problem then please write a feedback email first (use the “Help ▸ Send Feedback” menu item). I may not be able to help you, but feedback is never ignored.