I am using K2 for several years and it is terrific component for Joomla.
Problems rarely occur if you use a good template, not just the "Most Beautiful" which have many holes, and they cause problems with K2, and it is not the fault of K2, and the most common template.
So I advise you to choose the templates, not only in terms of their wyglądau, but also in terms of the quality of the code.
It has such a wide range that easily meets the needs of most users, because it has many similar features like "Google AdSense"
With an understanding of operation is so simple that each element is recorded videos to help, and if you want to make a difference, according to individual needs, the support from them is more than perfect.
I'm pretty demanding customer and I changed a lot, especially integration with Community Builder and I could count on the support at any time.
This is a great piece of well and cleanly written code.