I have been using K2 in some capacity since beta. I thought it was pretty good early on, but JoomlaWorks has really done a great job implementing the features that the community wants. The latest version is a fantastic replacement for the Joomla com_content. It works well as a CCK, a blog, or anything else you can imagine.
There are a few features that would be nice, but they seem to be coming in the next version which is already in beta.
Overall this is a great piece of software and one that I use on virtually all of the sites I build.