K2 has been a tough sell. I like what it does. But when I hand a project over to a client, they struggle with the plethora of parameters each category and item offer. It's made my life hard on several occasions. I've tried most of the CCK components. YooTheme's Zoo is easy to work with. SEBLOD is hard to learn but worthwhile for sites requiring custom content and presentation. It's not something I can just expect a client to take over when the site development is complete, though. I haven't had the opportunity to try SobiPro. I've used SOBI2 in the past and it worked very well.