They will never match the efficiency of a derailleur system, which is essentially a stripped down gearbox with no shell.
However I think they will offer a lot for certain uses like transportation and other "non-sporting" bikes.
Nicolai built a fat bike with one, which could be a pretty ultimate bikepacking bike.
![](http://brimages.bikeboardmedia.netdna-cdn.com/wp-content/uploads/2014/02/2014-Nicolai-Argon-Fat-Pinion-gearbox-beltdrive-fat-bike02.jpg)