I tried other CCKs including K2, Zoo, ContentBuilder and YooRecipe (it's for my cooking website), and I purchased the paid versions where applicable. I also bought the paid version of Form2Content because it has features I need for my project.
The other CCKs are good to okay, but do not offer the flexibility I need. I wish I would have tried F2C first because it turns out to be absolutely perfect for just about any content related project I can imagine.
Their integration of the Smarty template engine is nothing short of genius, allowing me to lay the content out exactly the way I want - not only in the browser but also in the generated HTML (think schema.org). And since F2C creates native Joomla! articles, I can use any content plugins I care to without regard to compatability issues.
I can't say much about support because, so far, I have not needed any. With their extensive and well written documentation, and the occasional search in their forums, I was able to figure everything out without asking for help. But from what I see in other reviews and within their forums, I feel confident support will be there if I ever need it.
My only regret is that I dumped money into paid versions of other CCKs, to the tune of several hundred dollars, before discovering Form2Content. If you're looking for the most flexible and most powerful CCK, you want F2C. Read the documentation, take the time to learn it... you will not be disappointed.