While in the past the BI market was strictly dominated by closed source and commercial tools, the last few years were characterized by the birth of open source solutions: first as single BI tools, and later as complete BI platforms. An Open Source BI platform provides a full spectrum of BI capabilities within a unified system that reduces the overhead for the development and management of each application, and lets the user feel like he/she was using a single BI solution. This paper proposes a comparative evaluation of three different Open Source BI platforms (namely JasperSoft, Pentaho and SpagoBI) aimed at understanding their current features, their future potentialities and their limits when adopted in real projects as well as a basis for research prototyping. Overall we try to understand if the open source phenomenon will be able to become a valid alternative to commercial platforms within the BI context.