When converting our music magazine from an ordinary php site with a mysql database to Joomla 2.5, we used import for K2 to import over 10.000 articles.
We had to edit our old article database structure to fit extra fields In K2 before using Import for K2.
The proses of editing the database was easy, after studying an export of a K2 item using Import for K2.
The proses of importing all our articles where complete after 2 hours of work. Which is an unbelievable time saver considering that it took approximately 5-6 minutes to copy / past one articles by hand.
You do the math. ;)
Not all is this extension easy to use, but you also decide what it will cost you.
The only negative thing I can say about this extension is that we had a little problem importing our images, but if you are a bit creative, you can also solve this problem yourself. However, I was aware of this problem before buying the extension.