I like Entourage - having been a long-time OE user, I found the transition pretty painless when I switched to X full-time over a year ago. It has lots of nice features and I find things like it's mail filters easy to set up and effective - though I don't get a lot of spam (touch wood ). It does a number of things Mail doesn't do - like encode files for Windows (which I MUST have, since I deal with a lot of Windoze users and particularly some on AOL who don't seem to be able to deal with non-MIME-encoded Word files)
With the addition of a few AppleScripts, (the one to sort my groups by last name instead of first being most useful) it has been great.