The migration framework is based on a very good intention from MS since it can usually be a hassle. It is quite competent but has one great lack being that it only handled system entities and not cusomtized entities.
The feature of customizing entities is one of the greatest USP (unique selling points) for MS CRM, and being a system in directed at mid-market it is more than likely that a customer has to migrate from previous data, if only being excel, access or Outlook. To not support customized entities is a great flaw that severly limps the otherwise fine product.
If you need to migrate to MS CRM 3.0 and you are using customized entities there are two choices:
1. Make your own speciall program that communicated directly with the webservice.
2. Buy a third party product.
In most cases I think the later choice is best, there is a very good product called ImportManager Studio 2006 Professional made by the Danish company CRM Extensions. link.
Not only is this product good for migrations it is also replaces the very rudementary import function found in CRM. Other than being able to import data to custom entities, it can also store an import set-up to be used at a later date and link imported entities to eachother (like linking a contact with an account).
It is only €1700 which is quite cheap when you compare it to the costs of writing your own program.
It will be interesting to see if MS updates the migration framework to handle custom entities. Until then, count on paying €1700 for a product that can.
CRM and SharePoint Consultant