Thunderbird 3 is underwhelming. But then, my email needs are pretty underwhelming, so it's a fair match. Generally, I ignore email as much as possible because life is too short. Thunderbird 3 helps with that.
There is nothing wrong with the search function. It's primitive and clunky, but I can always find anything I need, usually on the first or second go. I have an email filing system but the reality is that I don't use it - I just let stuff pile up, thousands of messages going back years, and if I need something I use the search function.
The new tabbled interface hasn't been thought through at all and has the look and feel of an early proof-of-concept alpha, not a finished product. I can't say that I care for it at this time. Maybe when/if they finish designing it and stop buggerising about with some functions in table and others (compose, for example) in windows. THey also need to address the close-tab "X" placement issue, and stop moving the search function around in the menu structure. Oh, and have an option - it should be the default - to search all folders in all accounts. This one account at a time that the present version inflicts on us is really rather sad.
So, there is lots wrong with THunderbird, but it is nevertheless (a) perfectly capable of doing what I need (just not doing it very elegantly), and (b) a much, much safer and more reliable email app for recommending to customers than Outbreak - for starters, with Thunderbird, when they hose their system (as most do, sooner or later) I can recover all their data with a simple drag and drop. And that is far, far, far, far more important than any of the minor detail level ease-of-use stuff we have been discussing so far in this thread.