I installed Thunderbird last week as a replacement for Outlook Express. The principal reason was security, a secondary reason was to reduce the amount of bloated Micro$oft software on my computer and a third reason was to see if it was a big an improvement on OE as Firefox was on Internet Explorer.

Pros

• Easy to install
• Easy to copy email from OE (and other mail tools)
• Easy to copy server and account settings from OE (and other mail tools)
• Superb junk mail filtering that probably means I can ditch my knowspam account
• Good keyboard navigation and keyboard short cuts (eg, ‘n’ takes you to the next unread message, ‘j’ marks an email message as junk)

Cons

• Impossible to copy OE message rules to TB filters.
• TB filters are defined on an email account basis so if I wanted to put all my CSS emails into a single folder for each of my 3 POP accounts I would have to define the same rule/filter 3 times. Under OE I would only have to have done this once.
• Can only define one SMTP server

And today I had a particularly annoying problem. The same newsgroup posting appeared 4 times, I could not mark 2 of these posts as read. Then all the messages from that group were marked as unread. Then TB crashed. This happened several times. In the end I had to unsubsubcribe from the group, subscribe again and mark all the messages as read. A particularly annoying feature is that when you restart TB after a crash it displays by default a feedback form wizard so you can tell the folks who wrote TB about the problem. This is probably a good idea but really irritating when you just want to get back to work. I switched that feature off.
I’ll stick with TB for a while longer but at the moment it seems to be a moderate improvement over OE, not a quantum one.