These hubs seem to be reasonably popular with certain recumbent bike and trike owners.
The main advantage is that it can be shifted between gears while stopped, has evenly spaced gears, and on 20" wheel bikes/trikes, avoids having a derailleur cage that clears the ground by about 1-2", which can be a bit sticky on off-road rides. (But if you are using more than one chain ring in front, you will need a derailleur-like rear tensioner to take up chain slack, which eliminates this advantage).
The big downsides are cost, complexity/weight penalty, noise in some gears, and about a 4% decrease in efficiency. Also, with fatter rear tires, the rear wheel may need to be 'dished' diffferently, and is sometimes a bit less strong than a non-Rohlof wheel. Finally, getting it serviced or repaired on a bike tour is a problem. Local bike shops can do at least a half-a$$ed job repairing/adjusting derailleur systems, but usually have never seen a Rohlof.
The 'other' German gearing innovation is the internally planetary geared Schlump drives,
http://www.schlumpf.ch/antriebe_engl.htm which have a single gear ring in front, with internal selection of a different gear range. This has many of the same advantages/disadvantages as the Rohlof rear hub system, but allows the equivalent of 2 front chain rings with no need for a front derailleur - front gear change is done by hitting the button in the center of the crankset with the heel of your shoe.
http://www.ice.hpv.co.uk/trikes/gears.htm