Hello fellow Magentoers :)
We at OYE decided to make a test migration project to get a feel of what migration from magento1 to magento2 is like. This article only describes data migration, porting of custom functionality and theme will be described later, when we complete it :) The magento2 docs page describes the data migration as a straight forward process, let's see if it is true.
The first problem was to understand if the migration tool supports moving custom table columns from m1 to m2. After some research, it turned out that it can’t do that. So if your sales_flat_order table has some custom column where you store some data – it can’t be transferred to m2 with the default migration tool functionality. I think this feature will be widely needed and it will appear in the migration tool soon.