I've been using FormidablePro for an internal web-based CMS. There's a lot of functionality, especially for the price. Overall the online documentation is pretty good, however the software itself is very customizable and it'd probably take years to document everything it's capable of doing. They have an fantastic support forum where you can search previous posts or ask specific questions. The quality of the support and timeliness of the responses easily rivals that of companies which charge hundreds or thousands of dollars a year for support.
Formidable Pro is one of the best tools I've used for WordPress and totally worth the minimal cost.