At first I was really excited. I installed the extension and went off to find tutorials on how to use it. This is where my problems with K2 begin and end. I found videos and text on WHAT K2 is designed to do, but not on HOW to actually do it.
I've searched and searched for the documentation and steps on how to perform steps, but I can't find them. If they are out there, I sure couldn't find them on the K2 site. I did find a good start on webcafeclub.com but unfortunately the author is still quite a ways from finishing her tutorials, though she has outlined the steps involved beautifully.
In my opinion, K2 is not for beginners. If you are a seasoned developer maybe the lack of good training documentation won't be a problem for you, however if you are a beginner I suggest staying away from K2 until a complete training series is available.
If I do find usable documentation on how to use K2, I will come back and share my findings if they make the extension usable.
Indeed, written documentation is lacking. But there is a wealth of videos in the K2 Community at getk2.org/community that cover almost any aspect of K2 administration.
Furthermore, I challenge you to devote some extra time to get to know K2. It's been built using very few "conventions" (aka stuff you need to figure out). It's mostly built out of plain logic. E.g. why should core Joomla article category options exist in the menus (???) and not the category itself? In K2, category options exist in the category edit page itself. Not somewhere else.
Like I said, devote some extra time and you'll see why so many people have given 5 stars to K2 and most importantly, you'll see why K2 powers today's biggest Joomla! deployments worldwide.
As Joomla! community legend, Amy Stephen, recently said on Twitter: "Thanks for K2. You picked up the ball for Joomla for a long time." https://twitter.com/#!/AmyStephen/statuses/156475956096286720