For myself, I have been using K2 for 6 months now, and I love it. Works perfectly. For the first time in the 5 years that I have used Joomla I don't have to mess with trying to integrate a million different components, modules and plugins to try and get comments, tags, etc... SO easy and clean.
My only suggestion for improvement is the double login at the admin side...maybe I can fix that problem? But I have not figured out how?
And, also the 'main photo'. Perhaps I just have not found out how or where...but I cannot seem to figure out how to make it 'adjust'. What I mean is, I upload the 'main photo' through the upload page (second tab I think) on the article submission form...and then in the article it always appears full size at the top, centered...and the article text can never go around it...it would be nice if we had more power over how that main image was placed and what size it would be, etc.: e.g. left justified, center (as it is now), right justified, etc... ?
Otherwise, great work...keep it up...and please know that most of us DO appreciate all the time and effort that goes into this free component.