I discovered this plugin less than a week ago and have since rebuilt my site around it. I won't name names but K2 replaced two other commercial products on my site (a comment system and a blogging system) as well as the built-in content system. Out of the box, K2 has virtually everything a publishing web site could need. This begs the question: Why wasn't all this stuff in Joomla in the first place? All developers and wanna-be developers should give this a try. However, non-technical people might want to stay away, especially if your time is limited. Although the free support forum is pretty good, there is almost no formal documentation for this product.
One more thing. Someone mentioned below that you will "probably" have to buy add-on products for videos and photo galleries. That's only half true. You will need add-ons, but they are totally free and open source.