I've fitted a NuVinci to my 20" Hammerhead as an experiment.
First off: The hub is really heavy. It's not really an issue when riding but as soon as you pick your bike up (to turn it round or whatever) you notice the extra mass. It's probably more noticeable as it's overhung at the rear end.
Secondly I've had a couple of issues with my hub.
My local bike shop built the wheel up with a 2x pattern at first, resulting in extreme spoke angles and broken spokes after about 2 weeks. The bike shop admitted that they should have built it up as a 1x and did a rebuild for free. The moral of the story is make sure you take the larger diameter of the hub into account when wheel building!
After about a month I noticed some oil weeping from the shift rod. After firing an email to NuVinci they replied that they had some problems and were dispatching a replacement rod. Less than 48 hours after I sent the original email a new part arrived by FEDEX (I live in London!). The modification seemed to be the addition of an extra O ring and the swap took about 5 minutes and cured the problem straight away.
Another month of riding later I noticed more oil, this time weeping from the wavy joint on the non-drive side. Another email to NuVinci got an instant response. Again said they had had some issues and that they would send me a replacement hub!
My hub has been a little problematic, but so far I have been very happy with the support from NuVinci. I'm guessing that the hub I got was a bit of a Friday afternoon job. Hopefully the new hub will be good unit.
Apart from these problems and the weight, I really like the hub. Shifting is smooth and the hub feels rock solid. The efficiency appears to be good, although the extreme ends of the range have a noticeable dip.
I guess that if this hub sells well NuVinci will look to re-engineer a lighter hub with a bigger range. Personally if they can get down to 2kgs and up to 400% I'll bet they'll have a real winner on their hands.