I have one and regret the purchase. The hub is likely well under 70% efficient. It feels like trying to pedal in tar. I converted a nexus 3 speed and will be going back. With the nexus I could pop my front wheel off the ground when taking off from a stop, not so with the Nuvinci. I also have to take many rests on the larger hills here when I did not need to on my nexus. I have about 150 miles on the Nuvinci and don't see putting many more, it's just that bad.
Now on flat ground the hub works as it should and is easy to shift.
I HIGHLY suggest trying one out first. At this point I feel robbed of $500.