In terms of funcionality the product stacks up pretty well albeit that you are pretty much on your own as to working out how to use and integrate it into your site. The article look better than core Joomla and for blogs and community sites work well. For a magazine based site, or for vertical sites such as property / rental management the look is less useful and for the life of me I can't get the articles to carry the global settings across (of which there are hundreds - this is both good and bad) and therefore I often ended up spending ages changing the article settings to remove all the bloggy user related stuff.
I also feel that their could be more layouts provided and that the core look of an article is pretty close to that provided by Joomla anyway. Sure it is nice to have the integrated gallery (I have never got this to work mind you) and the other bits and bobs but I would relly like to choose 1, 2 or 3 columns on an article by article basis as well as have some CSS options for article styling. Maybe I have missed this and this is all there - a manual might help here.
My biggest problem though is the system plugin which doesn't seeme to sit well with some applications and in chrome and IE8 cause the page to only part load. This makes the component pretty useless for me and perhaps others. I am not blaming the K2 plugin - it just doesn't seem to sit nice with some important core components.
Overall, if you have a simple blog site or community, want someting better than core joomla styled articles and can live with (or get round) some plugin conflicts then this is defo the product for you. If however you have some complex stuff going on and want more control over how articles are presented then you might still be looking.
What you can't fault of course is that it is free and for that it is to be applauded.