I'd love to use mail.app too, but there's one simmple feature which is missing and preventing me from using mail.app:

Having the possibility to assign a certain signature to a certain accounts. I (like almost everyone) am dealing with multiple email accounts and don't really want to waste my time by manualy selecting a signature on every single outgoing message.

What i also miss, compared to entourage, is the possibility to Auto-SYNCH (check) certain folders (i use IMAP) with my server. Currently mail.app either only synchs your INBOX or all folders which is realy bad if you're on the road. Entourage handles this very nicely with it's schedule setup. I also really don't like mail.app's combining INBOXES for all your accounts. I rather want my Accounts various inboxes be displayed seperatels - IMHO this should be a configurable OPTION and mail.app shouldn't force the combined inboxes on you.

Nuff whining.