As simple Joomla! user I was fully satisfied with standard Joomla CCK (if it can be called that). Few months ago I just found it's not enough and just doensn't fit my needs and requirements. So I've checked some free CCKs here and there and finally found K2. I was just overwhelmed by countless options, configuration stuff, possibilities, etc. However - after few hours with K2 Forums, some help-sites and dozens of teste, tries and mistakes I (at last) found this component really easy to use. It wasn't hard to learn, you just need to spend some time with it and play with anything you can find there (on some backup copy of your site, or fresh Joomla! installation for testing purposes only).
Now I can say that K2 is probably the best free CCK ever created for Joomla! It perfectly fills all my needs (and probably yours) - tags, comments system, social buttons, images, videos, extra fields and 10 billion/trillion/gazillion configuration options... looks scary at the beginning, but after few hours the best becomes the beauty, and the curse of complexity becomes a blessing.
I think everyone should give it a try - remember it's free, so it will only cost you some of your time!