Mozilla Thunderbird beta 2 was released not so long ago, so I decided I'd give it a shot.
I'm impressed. The GUI is much smoother, and much more slick.
Also, it can tag messages and use those, rather than depending on just folders for organization.
The message search is slightly more powerful, but still needs work. It's also not in an intuative location, which is the major drawback.
I didn't have any crashes, but I didn't run it for very long. If you want to test it, the download links are here: