This past fall I undertook my first content migration effort. This was to migrate an existing site from Expression Engine to Drupal 7. I immediately began looking at the Migrate module to learn it's use and here are some lessons I learned along the way.
First of all, be aware that migrate is a module for developers. It does have a user interface, but this is merely to run the import process. All the real action happens in the code, so you will need to be familiar with object oriented programming in PHP.
Five practical Drupal lessons learned during a recent content migration effort.