Did the conversion a month ago from PC's Outlook Express to Entourage... I needed to save over 3000 messages including attachments and an address book that had over 200 contacts.
For the addresses, I just exported to comma delimited and imported them into Entourage.
For the emails, I used EmailAlchemy (
http://www.weirdkid.com/). Well worth the price of admission, it transfered all my emails, preserving all the attachments and even my personal mail folders (about 25 of them). It costs $25 for single user use. There are other solutions out there as well, but this was the best I could find.