I’ve just switched to Thunderbird 1.5 and I’m impressed. There is no real noticeable difference in the overall look of the app but there are some changes that really make it worth upgrading from 1.0. The most noticeable difference is that it now checks your spelling as you type rather than only doing it when you click send. This will save a lot of time and effort and gives Thunderbird the only thing that apple’s mail client had over it IMO. The other thing they have finally addressed is the interface for setting up multiple SMTP servers. Each account now has a simple dropdown menu for specifying what server to use which is a big improvement to the hoops you used to have to jump through in version 1.0.

Basically, if you haven’t upgraded yet, get cracking and if you are still using something ghastly like Outlook Express or Eurdora get switching now, you won’t regret it!