Writing a migration from non a non Drupal site
It might not be the answer you are looking for, as it is not Migrate, but when I did this a couple years ago for a large client this is how I did it.  

First, I set up the Feed Import module and a map of column names to desired fields.  Second, on the various non-drupal sites that I was migrating to a single drupal install I exported the data to csv files paying careful attention to the column aliases so they matched the map.  Also, as it totalled a lot of rows, I broke it into several files to run them in batches.  Using an anchored feed import, I was able to run them in batches and subsets of data, to purge and re-import any set as needed, and to group all the difficult international names together by sorting my csv files.  

It took a day for every 10,000 nodes I imported.