I’ve been playing with Thunderbird on my Powerbook for about two weeks now, and I have to say I’m generally underwhelmed. My biggest issues:

1. Frequently, Thunderbird manages to show me a message in the mailbox list, but not let me actually see the message. For example, I currently have twelve messages in one of my folders, and while I can see the list entry for all of them, when I click on a specific one of them, the preview window is empty (well, save for the header information from the message that I was reading prior to clicking on it). Likewise, double-clicking that entry brings up a blank message window, and going to View/Message Source brings up a blank window, as well. And there’s nothing I can do about this — there’s no command to refresh the mailbox. Annoyingly, when it’s a message I really care about reading, I actually have to delete and recreate my entire mail account, then re-enter all the settings, and finally open the folder and read it. That all seems a bit much.
2. There seem to be major problems with offline mode. I have a filter on my mail server that puts all my mailing lists into a specific folder, so that they stay out of my face when I’m busy but are easily perused when I get a few free minutes. Since those free minutes tend to pop up when I’m disconnected from a network, I have Thunderbird set up to download all the messages in the lists folder when I tell it to go offline. Alas, it seems that the program randomly chooses which messages to download, and as a result, I’m frequently left with half of them being unavailable, and no network connection to remedy the situation.
3. If you add the previous and next buttons to the toolbar, they use the behavior that views the previous and next unread message, to much annoyance. This means that, instead of sending me to the next message in my inbox, Thunderbird frequently sends me to some message that was filed in one of my 150+ mail folders. The Go menu has entries for both options — read the next/previous unread message, and read the next/previous message of any type — so I know that Thunderbird knows how to behave correctly; the binding of the unread-specific functions to the buttons doesn’t make a lot of sense.

That’s what I have so far… I’m sure that Thunderbird will mature a lot in the move from 0.8 to 1.0, but until it does, it’s not the mail client for me.