I did Michaels method, dragging the folders from Outlook in to a folder so an mbox file was created. But when importing in to Mail I would say 70% of my 30 000+ emails was corrupted, no subject or sender info, and the error message "this message is in mime format...". After some days with try and fail I purchased the software called Emailchemy which allows you to convert a .olm file to several options including Mail 2.x - 6.x. I did that and then imported the folders and all the mails was now 100%. it was simply a mather of creating folders in Mail with the same name and drag and drop my emails in to new Mail folders.
In my case I have emails saved from my PC days years back, not only Outlook for Mac, so for me there just wasn't any other way for me to keep all my mails saved but to do this option. If you have tried all other options I can highly recommend this software. It did the trick for me.
Good luck! :-)