I have successfully migrated from MM 5.2 to Exchange 2010 and recently to the 2013 environment. I have always felt Avaya's voice mail products are lacking to say the least. My first go round with MM was not a happy one when we moved from an Opcom DIAL system in mid 90's. On the user side not much was lost, but on the backend there was so much functionality removed I couldn't believe so many tools were not available. I kept hoping Avaya would improve the product with their various acquisitions of other VM players over the years. The reality always seemed they were just trying to squash the better products by tabling them. I am not a Microsoft fan by any means, but the functionality of the Exchange UM products makes MM look like an old answering machine that uses tapes.
I will say the move to MS 2010 is easy if you following the integration documentation provided by Avaya. However the move to 2013 is trickier because of way Microsoft structured the new environment. The 2013 environment is not supported as of yet and still may require a patch for the Session Manager or MS 2013. Microsoft in their infinite wisdom decided to toggle between TCP ports 5065 and 5067 on the MBX servers every seven days. Basically the SM does not like this and if you try to build it out this way it will fail We are overcoming this hurdle by restarting the UM service on the MBX servers every couple of days. This keeps the port locked to 5065 because it is the default port and all is happy.