The downside to this is you can't have people watch you play. They're either going to see your screen or their screen, they won't be able to see both. I imagine games that use this technology will have a "traditional" multiplayer for when there are more than two people in the room.
Regular 3D just looks blurry to someone without glasses beacause the two images are pretty similar. What happens when the images are drastically different? It will be impossible to watch.
Grimey to his friends.