Well, I've been an Outlook user for ages... But the recent Firefox browser and Thunderbird Email Client is catching me eye. I'm using it temporarily as a test, and I can tell you that I'm getting more and more attached to them...

I too used to be an Outlook user for ages, but for the last couple of years I've been using PocoMail - www.pocosystems.com - my Italian partners tuned me onto it - it's got a lot of flexibility for techie geeks (scripting options), and tons of very cool features for something like $40

Never tried FireFox - I use NetCaptor - I love the way it keeps multiple windows open.