Giu Tae Kim
The user experience was easy and confortable. The firt days, I spend a couple of hours understanding the K2 panel. But it was so easy to work with, that I never asked the documentation. Also, the new demo page is very useful.
The support was (is) very good, whereas it is "free". Nowdays, the supports is much better than the past. You can go to the forum and check it yourself.
But, the BEST part is the features of the Content Consruction itself. The level of development and editing, to the way the articles are presented, is TOO FLEXIBLE. With only a few of html (most) and php knowledge, can create articles with custom extra fields, custom data, videos, images, and rich format content, like schema.org. The overriding of templates and modules, is REALLY SIMPLE and you can create unique styles.
The only CONS that I found is some things related of SEO, I mean there are a few of duplicates by page, and this is worse when using tags. But I bought the SH404SEF component, and this resolve my problems. Others things that lacks is an advance search (like live ajax finder), that include extrafields (but I think that there are some extensions that achieve this).
But I can't complain of K2... it's ismply THE BEST CCK out there... admit it