I'm a first time switcher (only two days ago!) and I grappled with this one for hours....
easiest way I ended up getting Outlook data...
was paid $9.95 for a add-in to mass export all vCards (contacts), and then for mail - I did a free trick.
Exported all mail to Outlook Express...
added my .Mac account in Outlook Express...
created folders on my Powerbook (in Mail) first...
created same folders in .Mac account (both in the web and in Outlook Express)
then in Outlook Express - dragged emails into the folders I'd created in there for my .Mac account (as I set it up as IMAP non POP), then automatically put then into .Mac account...
then clicked on the respective folders in my Powerbook (Mail) and voila they all instantly appeared.
So wasn't as painful as I was expecting.