I can confirm that it is (a) fast, and (b) currently a memory hog. This app is still early in development and has many signs that it was torn recently from the guts of the Mozilla App Suite. Those who know the strengths and advantages of Mozilla Mail already know and understand the value of this app, even on a platform with a pretty Aqua alternative for the average user (Mail.app).

I still do use Mail.app, and I don't believe any other mail application would compel me to just drop Mail.app in the Trash and empty it. That said, here's some reasons off of the top of my head to use Thunderbird:

1. Thunderbird has extensive offline support, and doesn't have to freak out or lock up when you're offline or float out of AirPort range.
2. HTML mail. I know, I know. Still, if you actually receive HTML mail that you want to see rendered correctly, OR if you have to create HTML mail, then know that Mozilla apps do this in the cleanest, most standard-compliant way.
3. Extensive option control and customizability. Way too much to get into here. Check out the prefs. There are numerous extensions and skins available online. There is much a non-programming user can do to make Thunderbird look and function in a personalized and effective way.