I've used Thunderbird and it's quite good as a replacement for Outlook Express. The spam detection is, in my experience, rather poor and marks a lot of genuine mail as spam. It also doesn't seem to integrate with Firefox quite as well as I would like - e.g. mailto: links don't open a new mail window etc. even though Firefox is the default browser and Thunderbird is the default mail client. However, I'm sure niggly little things like this will be fixed in the 1.0 releases.

There's nothing overwhelmingly special about Thunderbird, I mean it sends and receives email and allows you to sort it into different folders - what more do you want? It has standard stuff like an address book and newsgroup capability as well.