K2 best feature is the templating system. It's easy to understand and very mighty. You can use K2 as a blog, answering system, magazin style portal, for static content, as download system, even as a forum. As a middle class developer, you can understand the code of K2 really fast. It's no magic.
The support is very good. I have wrote Fotis much times over twitter, joomlaworks forum and the google code issue tracker and he responses very fast. And most of the problems was Beta related. The stable versions until K2 2.4.x have almost no bugs. I can't find them...
As you can see the flexiblity of K2, you can use it for 98% of the entire Websites of the world.
Where are the cons? I have'nt found any cons!
K2 is really a rock solid component. And when i have in the future three wishes from a genie. One of my wishes are, that K2 replaces the Joomla Article System!
Give K2 a try - it's a big step forward!