If you want to build rich, data-driven applications expressing the complex relationships that exist between data in the real world, I've not found anything that comes close.
Be prepared to spend some time learning to use it well. Fabrik is industrial-strength--it allows you to build your own database tables and associate data items in many different ways, so if you have an ambitious project and no background knowledge of database structures you might find it hard going at first. But there are some excellent tutorials, the documentation is clear and explicit, and the subscription levels are sensibly progressive.
Release Candidate 4 of Fabrik 2.0 seems pretty stable, and a quick browse through the forums shows the time, effort, dedication and, yes, humour that the developers are putting into this product. All in all, a serious product for serious people.