The Junk handling capability is especially welcome and powerful in this day and age of relentless junk mail. Search capability is excellent. Since the move to Thunderbird I have been exploring new features more than I ever did in Outlook Express. So this is not necessarily a Thunderbird vs Outlook Express report. Instead its just a re-assurance for anyone contemplating a switch.
I switched to using Thunderbird (I have been using MS Outlook Express for many years now) a few months back. Its look and feel is very much “Outlook Express like” with lots of cool features as a bonus. So if you are an Outlook Express user, the switch is painless.