Yes, it has a few nice features. Grouping all your accounts together in one meta-account is a good idea. The synchronising the IMAP folders by default is not. Isn’t the whole point of IMAP that you don’t have your mail on your computer. If I’m offline, I have no mail. Period. I cannot send anything then anyway, so what’s the use? Besides, I am always online.
But the worst of it all, is that Thunderbird 3 cannot handle large mail folders. I have a spam-account, which collects all spam, so that I can easily find false positives. There are some 100,000+ e-mails in the folder. And Thunderbird 3 is choking on it to the point of unusability.
Luckily, you can choose which folders to include in the ‘smartfolders’ and make it behave a little better. For now, I will use it and give it a grace period of a few weeks.