i agree with all of the counter-posts regarding the Rohloff Speedhub...

i have considered it myself and will probably put it on the next bike i buy.

i have heard ZERO reports of durability being a problem. actually one of the big reasons many buy it is durability -- the other is you only need one front chain-ring so you can use a strong double-bash guard AND with a straight chain-line you don't loose a chain and rarely break a chain... it's getting very popular in the downhill and Freeride scene although you obviously won't see it on the low-end bikes.

weight is a factor, but if i remember right, on a MTB the Rohloff system weighs less than an XT setup although less than XTR... although i don't run XTR on my bikes as it is expensive and XT is usually more durable.

the 2 major complaints from the mountain bike crowd:
1) weight DISTRIBUTION: you have more weight on the rear than a typical 3X9 setup so it changes the handling - guys often say they have to "relearn" to bunny-hop
2) INITIAL cost -- almost everyone i know who has Rohloff says they SAVE money compared to an XT or XTR system as the maintenance/durability is so high --- for example, i replace almost the entire drivetrain once per year -- cassette, front chainrings (with my chaingaurd the big ring lasts 2 years), chain multiple times -- and i break a rear deraileur about once a year and front every 1 1/2 years... --- this means in year 2, 3 and so on a high-mileage/heavy abuse rider like myself probably SAVES money compared to a mid- to top-of-the-line deraileur system. (unfortunately most pre-built bikes come with Deraileur systems so you get a discount and have to upgrade or build your own Rohloff system)

as to the initial topic: shaft-drive if it could be made cheaply might be good for "cruisers" and commuters, but for high-performance stuff the chain seems to offer more advantages b/c of efficiency.