Having played around with it for the last day I've been particularly impressed with the level of control the new search feature offers. Once you enter something into the search bar, which is dominant at the top of the interface, results are ranked by relevance and displayed along a graph for a visual time line of when messages arrive. Users can then filter results using a series of parameters and save the search as a virtual folder.
It works really fast and is much more comprehensive than Outlook's native search feature -- though in all fairness Xobni has been offering a similar experience for some time via a free plug-in. I didn't care much for the tabbed user interface at first, but have come to appreciate the ability to open several messages in one fell swoop to check on later, all without cluttering the desktop with multiple Windows. Numerous available add-ons can also be displayed as tabs so you could load Google Calendar or Wave right within Thunderbird, for example.
The program's account configuration wizard also received a critical usability enhancement to simplify the initial setup process -- no more asking users for their POP/IMAP, SMTP and SSL/TLS settings. All in all, I came out very impressed with what Mozilla Messaging has done here, and might actually end up switching back to Thunderbird after using Microsoft Outlook for more than 2 years.
If you rely on a desktop client for managing your email you shouldn't hesitate to at least give Thunderbird 3 a try. Mozilla has the full release notes here and the program is available for Windows, Mac, and Linux systems.