I used Thunderbird in an XP office instead of the dreadful Outlook Express. My favorite features:
- Spam Filter: it learns as you go, it requires no extra install and it works for me
- Search Folders: I work at a non-profit where many of us are involved in multiple projects. I'd get an email from Bob that had to do with Project A, B, & C, and one from Mary about B & D, and one from Joe about A & D (I know, the poeple in my office have ATROCIOUS email habits). However, I could throw all my mail into one big folder and then use search folders to find stuff. One for each co-worker. One for each project. Fast & Clean.

CONS:
- As soon as you want to do something other than email, TBird starts to suck wind.
- It's easy to try and make it more than it is or should be by installing every extension out there.