Thunderbird was my mail client of choice on a Windows machine and I was disappointed with the OSX version - crashes at every possible opportunity just like Firefox - what a load of steaming crap.
Its probably a good thing anyway because I want to use the default address book built into OSX as I can sync all my contacts with my iPod. The default address book integrates will with OSXs Mail.app rather well. Mail.app is actually not that bad once you get used to it.
So yes - the tools for switchers gallery will also include tools that you would probably want to avoid. In this case - stability of Thunderbird may improve over time but for now stay away. But if you insist on trying it out...