I've been building Joomla sites for about 5 years and as I've looked at different extensions, a number of them indicated that they were compatible with K2. While this mildy piqued my interest, I never justified the time to research this any further until now.
It turns out that K2 could be a very useful addition to Joomla.
K2 replaces the functionality of the com_content component which is the main engine behind articles in Joomla. The thought of this actually scares me quite a lot since articles are the fundamental building blocks of Joomla but it looks like K2 offers some benefits that are hard to overlook.
What K2 offers is a much richer article structure which allows you to have more fields and parameters for an article. With the default Joomla article, you can give an article a name, give it some intro text, add a readmore break and type the content of your site and add some pictures.
K2 gives you the ability to add a lot more parameters to your site which can help to standardize the structure of the content on your site which in turn makes it easier for content authors to produce content with a consisten look. K2 allows you to define additional fields for an article which may be displayed along with the regular site content. For example, if a section of your site includes book reviews, you might want all book reviews to have a section that identifies, the publisher, number of pages, age demographic for the book, and a link to Amazon to buy the book. K2 allows you to do this.
I haven't had any chance to play with K2 yet, but I believe the ability to apply more structure to article data will be tremendously helpful in making it easier for less technical users to maintain and produce site content.