Seemed to go pretty quickly from Release Candidate to full version, but still it's pretty cool that Thunderbird 2 is out. I've been using Thunderbird 2RC1 with an IMAP account for the last few days and couldn't be happier with the product. Unless you're locked into an Exchange environment, you should definitely check out Thunderbird -- Gmail users will appreciate the quick setup for Gmail accounts.
Note that for some reason the release candidate update engine isn't working right now, so you may want to download the application directly to update.
Click the picture below to download.