I'm forced to use MS Outlook at work. I don't use it at home, and I refused to use it at my last place of work. Over the years that I've not used Outlook, I've never had email virus problems, while colleagues have received and sent them without even realising. Since I've been forced to use Outlook I've had time to actually evaluate it, rather than my previous dismissal without first hand evidence.
I can say that Outlook is possibly one of the worst pieces of software I've ever had the misfortune to use. It's slow and clunky. Insists on sending everything as HTML and raw text. Forces me to see other people's appalling taste in screen fonts and colours. Encourages people to put their answers at the top of the message, leading to huge messages full of repeated text. It doesn't even do in-line spell checking, I have to run that as one extra step! The list goes on and on.
Okay I'm only using Outlook 97 (8.x), but I'd have to say for a product claiming to be on version 8, it's pretty immature, and needs a darned good sorting out. I find Eudora, Pegasus or even Netscape Mail better products that Outlook, I think I even found KMail 0.x a better application and I only used it for a few days!