I think this is a great idea. Unfortunately, several of the images have alignment issues and other problems. One of the mockups thumbnails covers up the user's thumbs with the screen, for example: http://smartmockups.imgix.net/47_bg.jpg?fit=crop&w=360&h=270...

Also, have you thought about doing some basic colour balancing on the uploaded images to match the environment in the image, or overlaying some subtle alpha-channel reflections? (Both of which would greatly increase the realism)