Malte,
The OpenACS API seems to be well documented. What I didn't have access to was the ERD and all the other data model documentation. Now that I have both, I'll have everything I need to do a "full examination". At the same time, I may decide not to use certain aspects of the API and create my own implementation (different language, bypass the object model, etc.). In that case, I would want to "dissect" the data model a bit more.
In summary I would say that in order to do a thorough evaluation of any toolkit both the API and the data model should be examined.
