That's an amazingly good efficiency. I tend to take manufacturers' ratings with a large grain of salt, but if they're claiming 97% it's got to be pretty decent in any case. If you can get past the weight it's a pretty neat setup.

I'd love so see something like that with an automatic control - rather than the twist grip selecting the drive ratio, let the twist grip control the cadence. If you start spinning too fast it'll lower the ratio to keep your cadence in the target zone, and if you start spinning too slow it'll raise it. If you want to adjust your cadence you just have to twist the grip.

It's probably still not something I'd buy - I love hearing the derailleur snick back and forth when I change gears.