Thunderbird is good, but then there is nothing particulary wrong with Outlook 2007 IMHO. Thunderbird did used to have a memory leak where leaving it open for a few weeks would cause issues, haven't used it since I got Outlook 2007 though.
The big advantage Outlook has is the built in calendar and that it easily sync's with windows based mobiles, which is the main reason I use it.
The one thing that is missing in Outlook is the ability to auto BCC e-mails that you sent out. You can auto CC them, or hack about with some vbscript which isn't very elegant. Wish they would just add a BCC checkbox into thier rules.