Mozilla Thunderbird is now available as it's very first milestone release. For those of you who have been dubious of getting builds, this milestone release should be something you would feel comfortable trying out.
Installation on any platform (currently only binaries for Windows, Linux and Mac OS X are available) involves only unarchiving the binary archive and running the executable - no messing with the registry for Windows users. Uninstallation is as simple as deleting the Thunderbird directory, and your profile directory too if you don't wish to keep it.
If you haven't yet, try it, if not for it's coolness factor, then for it's Bayesian spam filter.