Hi Elisa,
I have started a thread here called - Ariba Easter Eggs- to share the 'undocumented features' , Limitations and 'not so good surprises' we are facing during our implementation of Ariba..
Overall, the product is poorly documented and with a significan inconsistencies between the available documented information.
Though it is now a SAP product, the integretion with SAP is at best 'POOR' 
We have Ariba consultants helping us for the project and they are really knowledgable, however, the product fails to stand on its own two feet .
Regards
Rahul