I use Entourage except on my work machine, where I'm forced to use Mail.app.
I am not a big M$ supported or anything, it's just that Entourage is much better then Mail.app in my opinion.
They do tend to share the same features, and I have not doubt that Mail.app will eventually catch up to Entourage one day. But for now, Mail.app just doesn't work as it's supposed to.
An example of what I'm talking about.
Just today I was asked to send a file to a client via email. Without thinking about it I attached the source file, instead of the compressed (and much smaller) final file. I clicked send.
Well you can imagine my surprise when I came back from a bathroom break to find that Mail.app was still trying to send the file. Using the "Activity Viewer" (which is a complete rip off of Entourage's "Activity Viewer") I realized my mistake.
Well, thankfully Mail.app has a nice little icon that says "Stop", presumably to stop the corresponding action from completing. Well, guess what, it doesn't work. The button is dimmed. Apparently there is no way in Mail to cancel a sending email.
I had to force quit and relaunch the application, to find Mail.app attempting to send it again. I had to force quit a few more times before I could finally move the email to "drafts" and delete it from there.
I don't like having to trick programs like that. I see that as a flaw in the programming. When you display a cancel button, guess what, it should cancel.