I have just recently switched to Thunderbird so see how development has progressed. At this point I am reasonably impressed. The setup for POP/IMAP email accounts was the easiest with which I have ever had to deal.
With a simple extension I had communication with Unity. Not shown in the image below is the indication of new mail in my various inboxes.
I have not had the opportunity to experiment any newsgroup reader functionailty.
Like Evolution, I like the fact that Thunderbird provides me access to my GMail contact list. Additionally, I like the fact that Thunderbird works with tabs, one of which is my Google Calendar; not a reasonably close facsimile, but the actual Google Calendar webpage.
In the past I have enjoyed KMail, Evolution, Claws Mail, and Thunderbird. Currently I am very much liking Thunderbird. For a list of the features of Thunderbird 3.1, refer to: http://www.mozillamessaging.com/en-US/thunderbird/features/#easy-upgrade