I've been experimenting and using ZOO for over 2 months now so I can say I have a pretty good grasp on the system.
In my opinion out of all CCKs available for Joomla, ZOO is certainly the most elegant of all of them. It is intuitive, easy to use and once you get how the system works it's fun to build pages using the elements. Templating is pretty straight forward and overall development is fairly easy.
From the presentation on Yootheme's homepage and the thorough documentation it's clear that a lot of care and attention went on the development.
The problem with Zoo, and it is a major flaw for a CCK, is that once you get past the submission and the display of data on the front-end you dont really have much control over the data. Display and filtering options are really limited.
Things get really bad once you take on account the experience of a front-end user. Yes, you can submit data and it will show on the front-end, but if you want to have more control or any other special relations besides that then you're out of luck.
Let me give an example, let's say we have a business listing website and users can submit items from the front-end. Let's also say that I want all the authors to have a blog if they want. Then trouble starts, because if you want to display those blog items teasers inside the author's business listing, there's no way to filter that data based on the author as there's no way to put a link to that specific author blog page since there's no way to filter authors.
And that's where the problem lies. If you want most of the content of your website to come from front-end submissions and later you want to give those users all kinds of possibilities for submitting content, zoo is really not the ideal option. The base of the cck is really solid, but this is extension leaves a lot to be desired on an author basis level.
There's not really much room for customization from the front-end user besides editing the data. There's no way to create multiple applications that are related to each other and in my opinion that's a pretty big flaw on a cck.
On the flip side, if you're not interested in complicated uses of data, and just want a basic sumbission form or if most of the submissions are going to come from the back-end then by all means use Zoo. It's the easiest to learn and use and for simple websites I totally recommend it.
On the other hand, if you are planning complex submissions and want better control for front-end users then I don't recommend Zoo at all. You'd be much better off trying Cobalt or Seblod.
Dont let my score fool you, I really love this extension. As I said, the system is intuitive and elegant, it's extensible and it works for simple purposes. The problem is that it is an expensive extension and 3rd party extensions providers like Zoolanders are really necessary if you want to do anything beyond the basics or want integrations with other extensions further adding to the cost.
I really hope Yootheme will bring new features with future zoo releases, specialy more control for the front-end user and more control over the data that is submitted. If they deliver, then this cck is going to be best there is for Joomla, but as of now as a cck it is not the best.