Why are some commenters so dramatic and why is the rating so low? Overall this is a nice application with some smart features. When you type the word attachment or attach, it notices and reminds you to actually attach something. This can be annoying sometimes, but it can save your face some others. You can view very easily all the attachments you ever received. It allows me to delete permanently messages from gmail rather than archive them.
In general it is pleasant to use. I think however it has some bugs, for example it shows me that I have unread email "On My Mac" but when I click on the folder it is empty. Also the memory usage is insane (370MB compared to 110 for Mail.app). However the memory does not seem to be increasing with time (or I didn't wait long enough). Finally, it takes a long time to quit the application. It takes like 10 seconds to quit after pressing Command+Q. Other than that the application feels pretty responsive.

I am not going to buy now mostly because of the memory usage and the "On My Mac" bug. But I think it has potential and I will keep it in mind for the future.

The usability is very nice. It contains all of unix emacs along some mac/cocoa improvements. However the speed is horrible. Compared to the unmodified emacs and also textmate it takes ages to load, especially if you pick your own color profile. You would think that cocoa would make it more "native" and thus faster, but this is not the case. I deleted all my preferences and still the startup time is ridiculous on a macbook pro. For someone who does a lot of small edits this is not very practical. For this and also the bundles I switched to textmate.