Not sure how you all feel about the big G... but I love Google and everything free. I've been using Gmail for almost a year now, routing all of my other emails into my Gmail inbox. It automatically collates messages in a thread so you can refer back, it has integrated chat for when you need a quick back and forth exchange for someone. But the one thing I have been missing from switching over from Entourage was a Task feature. I had to revert to a pen and paper to do list for a while. But I am back! Paperless yet again! The new task gadget is great. You can create a task while you are inside a relevant email. You can connect it to a due date... organize tasks into separate lists (great if you have multiple clients like I do.)
Bottom line... I love it. And I love Google. Gosh Darn-it... I love the whole world right now.