The three stars is for The Animatrix as a whole, which is a mix of animated shorts ranging from wonderful to pretty horrible. The Final Flight of the Osiris is the short the most closely tied in with the movies. It serves as a stepping stone for Matrix Reloaded by showing what happened before the movie's events and how the characters came to know about the danger to Zion.
Like Final Fantasy, Final Flight of the Osiris suffers from too much static realism and not enough animated realism. In other words it's pretty to look at but not so pretty when it's moving. However the "fan service" thrown in should keep some people entertained, and the impending doom of the Osiris and the final fate of its crew and the messanger sent into the Matrix keeps this short action-packed and gripping. But you'll find better shorts on the rest of the Animatrix collection.