Horses for courses I'm afraid. If you're a long term Entourage user and you're aware of and happy with countering the constant attacks on Microsoft coding why change?
I've switched to the built in mac mail client exclusively. It's spam/junk mail training works very well and remains nicely configurable once switched out of training mode.
The address book is of course integrated into your other applications as well - not just Office. Thus highlighting a pic in i-Photo and then clicking on the e-mail icon automatically puts the .jpg into a blank message form and the address box is highlighted for you to enter the first few letters of the recipient. Play with the usual keys and you will find you can place multiple photos in an e-mail with a single click.
Of course the photos are fully visible in your mail and you aren't looking at file names on attachments to check you've included the correct shots.
Haven't found a flaw in it and, as you can imagine, running the site gives me a whopping mail load and an address that gets into an awful lot of naive, unprotected ppruners address books.
Others may have had a different experience and can counter with their thoughts. One obvious reason to use Entourage would be if you're a serious Word user and need the mail merge facilities along with a carefully developed and transferred address book.
Regards
Rob