And Peter gives excellent quick responses to comments posted on his forum.
Yes, it's free, but you really ought to pay for it-- it's worth it!
The best in that it is an obviously well developed product, with oodles of features and functionality and a devoted user community.
The worst in that it has (as is all too common) next to no documentation. The sole piece of documentation, a "quick step-by-step guide", tells you in 12 rote steps "what" to do to get K2 basically-kinda'-sorta' up and running, without touching on whys. OK, I've got extra field groups and extra fields. That's nice. What are they for?
I know that they say that this product is not for Joomla beginners (which I'm not); but more honestly, it should say that the product is not for anyone who does not have 6 to 12 hours to spare mucking about and kicking the wheels to figure out the workings of the system. Judging by the mostly positive reviews here, there are a lot of such users who are willing to do so, and more power to you. I’m not one of them. I resent having to re-invent the wheel, 'cause it's so inefficient and unnecessary, and a waste of my time.
Deleting K2 from my system for now, which is a pity, 'cause it does seem good; I'll check back in a six months or so to see if anyone has documented anything yet.