Just found a nice and personal Mailplane review. It has been written by David Coldrick from Tasmania:
There are a number of differences in my day since I left the corporate life a year ago. One of them is that I now use Gmail exclusively, instead of Mac OS X Mail: I found Gmail via Mail rather clunky, and Gmail kept coming out with more good stuff. But I missed the integration of a Mac desktop app: Mailplane provides that in spades, while retaining the look and feel of Gmail.