Coldfusion - MsSQL to Drupal Mysql
I would definitely start by looking at the Migrate module. It has a plugin for MSSQL. Then, with writing some code to extend the module, you can tailor your migration to fit your needs. You can define data sources and targets, extend the migration classes, hook into the migration function, run batch migrations to test, and many other cool stuff. It is also well documented and has a few examples and case studies. You can read the documentation here on how to use and extend the Migrate module to build and/or enhance your migration process.