I use Outlook on a PC at work and used to use Entourage on my Mac.
I switched to Mail because I like it's simplicity. And Entourage was overkill for what I needed. I also really like iCal and prefer it over Entourage.
The one thing I don't like about Mail though is when sending a picture to someone. It puts the picture in the body of the message rather than keeping it as an attachment. I know there are shareware applications that allow it to stay as an attachment but I don't really want to have to buy something extra. I'm hoping this is 'fixed' in Mail for Leopard.
If you're used to Outlook, Entourage is great, but if you don't need all the bells and whistles and you just need to read your email, I prefer Mail.
Cheers.