I have worked with SP Transfer by cyend and it is doing the job pretty well.
But depending on the conditions, I also do this by using a desktop database management app that allows me to perform efficiently database operations.
In my mac I use Sequel Pro.
I can connect remotely via SSH to more than 1 DB server and have opened windows for the databases I am working on at the same time. I can easily choose any records from any table I want, and import/update records to the other table.
There are many cases that I consider this method to be much more efficient and faster for me.
In the case of copying articles, we assume that the overall db structure and categories organization is the same between the 2 tables, otherwise there is going to create a mess with the assets and the categorization of the articles.
But someone needs to be careful and know what he is doing, because we are dealing directly with the databases.