I’ve been using Thunderbird for quite awhile now and even though it’s still technically quite a ways from it’s official version 1.0 release it’s already robust enough that I can’t imagine going back to OE. My reasons for switching were two-fold:
First, virus writers tend to exploit OE the most by having the virus access the address book in OE and use that to send itself to everyone listed therein. Thunderbird uses it’s own address book and is thus largely immune to that sort of thing. At least until it becomes more popular.
Secondly, I switched because of Thunderbird’s built-in “junk mail” filtering. This feature alone makes the switch worthwhile as it uses the Bayesian method of SPAM filtering that assigns words a probability of being part of a SPAM email. The more words with a high probability in a message, the more likely it’ll get tagged as SPAM and put into a junk folder for you to look at later or ignore as you see fit.
You spend a few days teaching this filter what you consider to be SPAM by selecting Run Junk Mail Controls on this Folder from the menu and the program will go through and “guess” what is and isn’t SPAM. You go through your messages and make corrections using the JUNK button to un-mark messages the program incorrectly guessed and to mark messages the program missed. Run the controls again and again make any corrections needed and on the third run the program will probably not change anything as it has “learned” what you consider to be SPAM at that point. After a few days of doing this you’ll find that Thunderbird gets pretty accurate pretty damn quickly and then you can turn on the option to have it move these messages to a Junk Folder at the time of download.
At this point Thunderbird is catching easily 95% of the SPAM that comes into my inbox and I haven’t had a false positive in weeks. What more could you ask for for free? The only reason I’m still using Internet Explorer over Firebird as a browser is because I haven’t find an equivalent for ieSpell for Mozilla yet.