I installed a Schlumpf mountain drive on a Cannondale MTB I set up as a city/commuter bike. I left the 7-speed derailleur on the back and so ended up with 14 speeds and a gear range of about 17" to about 95".
The Schlumpf works very well, exactly as advertised. It is easy to shift ranges and has never once--in five years--jumped out of gear.
There is only one problem with using the Schlumpf with a derailleur: when you downshift into the low range on the schlumpf you usually want to simultaneously go from the lowest to the highest gear on the derailleur, which is a bother on steep hills. Using a Schlumpf with a geared rear hub would probably make that less intrusive.
The other problem, of course, is that I still have a rear derailleur to go out of adjustment.
In fact, the Schlumpf made me such a fan of 14 usuable gears and of planetary gears rather than derailleurs that I've got a Rohloff bike on order!
But if you don't want to make the jump directly to a Rohloff, I can recommend retrofitting a Schlumpf mountain drive to extend your range and simplify your shifting.
