I love Joomla!, but one of the missing features which I found most important was a useable CCK with template capability. For most sites, I would use mtree as a directory or product listing which worked very well, but you can only have a single installation and the categories always link to the same parent. It is meant to be used as a directory, not a CCK. K2 was released at the perfect time. I had two large websites that needed multiple directory style listings: Team listings with profiles, video gallery, article listing, photo gallery, product catalog, event calendar, internal file repository, and a support section with downloads. K2 was used for both sites and met all of the requirements. The ability to use templates for each category, style custom fields and create multiple "do anything" modules (multiple module templates) with custom templates is EXACTLY what I needed.
A big plus is the fact that all sections of the site, which would usually be multiple components, are managed from the same admin section. This makes client training a breeze! I can't wait for future releases!