Initially I used this component tool in my home PC test environment. At that time my environment was based on a version of Xampp 1.6.3 which delivered PHP 4.4.7 and PHP 5.2.3. For some reason mtwMigrator just would not work properly for me. It gave me error messages I no longer recall and did not migrate anything. So, I set it aside.
Over the period of the following week I spent at least 24 hours trying to get the main supported migrator to work for me satisfactorily. Nothing seemed to come out the way I wanted it to. Either the regular migrator left things out or did not work at all. Some of the problems were related to errors on my part, but many of them had no explanation at all. It was most frustrating to go through the entire process of generating a new migration script and trying to use it to build a new site based on Joomla 1.5.1 only to receive the most unhelpful message of (to paraphrase here) "your script failed!" This happened to me time after time after time.
Yesterday I upgraded Xampp to version 1.6.6a. That environment provides PHP versions 4.4.8 and 5.2.5. I again tried to use the standard migrator under both versions of PHP. I got the same kind of errors I have been receiving before. On a whim I decided to give mtwMigrator another chance (same version I had used before, 0.0.4). Running under PHP 5.2.5 it worked like a charm! Everything I needed was migrated perfectly.
I really prefer the approach used by mtwMigrator compared to the standard migrator component. The standard migrator component tries to make a replica of your old site reincarnated as a Joomla 1.5.1 site. So, if you have eliminated some standardly supplied modules from your old site (either because you do not use them or, in at least some of my cases, tried eliminating some in the hopes of reducing the workload on the migrator), they won't be in your new 1.5.1 site. Similarly, capabilities you had in your old site, but which are different in 1.5.1, might not be re-created in your new site. For example, in the old site I had a standard core-supplied module called "Syndicate". In Joomla 1.5.1 the standard replacement module is called "Syndication". The use of the standard migrator in building my new site left that capability totally out.
But with mtwMigrator you build a standard 1.5.1 site first, so all of the core-supplied modules are provided. Then the mtwMigrator simply transfers over those users, modules, sections, categories, etc., which are unique to your old site. You can then decide later which of the core capabilities you want to keep, simply un-publish, or eliminate. A much more logical and helpful approach, to my way of thinking.