As software grows more complex, software developers' productivity is increasingly defined by their ability to effectively reuse code. Even APIs (application programming interfaces) and other code explicitly intended for reuse are often difficult and time consuming for developers to use. This paper describes the user-centered design and evaluation process we evolved in redesigning SAP's BRFplus - a business rules engine, whose API was created for platform development, but which is now also increasingly being used by application developers - even though it was not initially designed with their specific needs in mind. Our API redesign attempts to take both the initial as well as the new emergent user requirements into account. A usability evaluation of our proposed changes to the API suggests that our user-centered design process was successful in helping to create an API that significantly improved users' productivity and better matches the different users' needs.