I have just finished listening to the Audio Book of Ender's Shadow and I was completely impressed with the professionalism of the production and the quality of the writing.
Ender's Shadow is not a sequel or a prequel. It's the same story as Ender's Game but told from the viewpoint of a different charactor, Bean. I was so amazed at the new insight into the original story that I found myself rereading passages of Ender's Game.
The quality of the production on the audio book is amazing. The producers wisely decided to use a full cast to read the story. When the story switched viewpoints and during certain dialogue pieces, it helped to keep the charactors straight and the audio book lively.
Orson Scott Card decided to add a special treat at the end of the audio book. On the final CD, there is a passage where he explains what has been holding up the Ender's Game move. (Yes, there will be an Ender's Game movie) He tells you of the obsticles that had to be overcome, how he overcame them, and why there will be an Ender's Game movie out soon. The insight that he provided is amazing and surprising.
If you are wanting to read Ender's Shadow, I challenge you to listen to it on audio book instead. I listened to it on at the gym and in the car. I'm sure that you will feel the same way I did; that it was well worth the unique experience.