The address book intergration works well and I'm happily using those adresses. But I noticed some more changes, which are worthy mentioning:
- Thunderbird is now a native cocoa app
- The coolest thing ever though, is how tabs have been introduced, keep multiple mails or inboxes or feeds or ... open at a time is really nice. Tabbed browsing for mails: awesome!
Even though it's beta, it's been running just fine, so I recommend giving it a try!