Calling ThunderBird fast is wrong, imho - it has the same bloaty-slow startup feeling that FireFox has (I bet it's the whole XUL interface deal), and moving large amounts of messages between folders is slow - because of the retarded all-text MBOX format used.
Yeah, I should have qualified that. STARTING ThunderBird takes the same amount of disk thrashing as any other app (Outlook springs to mind) especially if you have a bunch of TB add-ins installed, as I do.
But once it's running it's quite snappy. Even when background stuff is running, like filters or PopFile. I have four email accounts set up, two of which are GMail and HotMail, and even they query/download quickly.
I haven't noticed any lag when moving messages from folder to folder. It takes 2-3 seconds to move a coupla hundred emails from Junk to Trash, for instance. But I would think that's acceptable, given the amount of work.
The main problem (which is why I think I rejected it last time) is the lack of export options.
I found this page
describing how to export from TB to Outlook Express, then from OE to Outlook. I also found some add-ins that automate the process, but they prove "incompatible" with my ragged-edge version of Firefox+TB.
True, it's cumbersome to export from TB-->OE-->Outlook, but that would be a one-time fall-back option if you decided TB sucks, right?
BTW, the Lightning scheduling add-in is quite nice, and makes me (almost) forget about Outlook. But if you prefer a more robust scheduling option, there exist add-ins to support Rainlander(sp?) and other dedicated apps.
FWIW I am not a rabid TB fan, just happy that I've got direct access to my email via text editor if things go kaboom. At one time I wanted to write something that would parse email, and Outlook kept getting in the way. Once my .PST files started to unravel that was the impetus to jump ship.