Originally Posted by Dahon.Steve
Weight is only one of the problems. The major issue with the Nexus 7 or AW-3 is the inefficiency from the internals. This makes it feel like the hub is much heavier than the actual weight and the SRAM dual drive when shifted in 3rd gear is very inefficient.
I also happen to believe when the weight is all shifted to the rear hub, it feels like you're carrying an additional two water bottles!
From the scientific point of view, indeed internal gear drivetrains are less efficient than derrailleur drivetrains. But, I use internal gear drivetrains for several years and I didn't notice a difference on efficiency. From the bicycling science book a scientific comparison gave an average difference of 4 to 5 percentual points between derraileur drivetrains an internal gear drive trains.