have been using Thunderbird and Firefox for long since their 0.x releases. Don't use IE unless absolutely necessary.

Thunderbird 1.0 is simply awesome with features like Group By Sort, Saved Search Folders, Image blocking, the RSS reader. Good interface with goody functionality. Cannot compare with the MS Outlook cos have never tried using it. I have heard that Outlook has calendar support, but I don't use the calendar for anything, so doesn't make too much sense.
Image blocking prevents images in HTML mail from auto-loading. Speeds up mail retrieval and uncluttered mail.
The RSS reader is amazing. Now I get to see updates to Blogs, news headlines from multiple sources (I read news from rediff , tried indiatimes but their news sucks as always) . All without having to fire up a browser.
I am yet to enable the junk filter on 1.0 but it worked well on 0.9x versions. I am sure it should work as well as 0.9x, if not better.

There are some very minor fixes needed on the s/w. Nothing that is troublesome.

What r u waiting for? Zip to http://www.mozilla.org/ and learn for urself. And try Firefox too, u'll never want to touch IE with a long pole.