Some quotes from the websites of Dynamic and Sussex (which appears to be the same or interconnected companies, that isn't clear);

"Our unique design minimizes weight and increases efficiency and performance far beyond any other shaft drive on the market..."

"How durable is the shaft drive?
"Our shaft drive is designed to have twice the life expectancy of a chain under comparable riding conditions."

"What maintenance is required for the shaft drive and internal gears?
"With our shaft drive, the only maintenance required is grease added to the front and rear gearboxes on a periodic basis - typically every 500-1000 miles. The frequency of this lubrication will depend on the amount and type of riding you do, and the quality of the grease used. More aggressive riders and heavier riders may need to add grease every 1-3 months, while more recreational or occasional riders may only need to add grease every 6-12 months. Grease is added to the front gearbox through a standard grease nipple (zerk fitting) on the underside of the gearbox using a grease gun. Grease is added to the rear gearbox simply by removing the rear plastic dust cover and applying grease right to the gear teeth. Typically, greasing the shaft drive takes less than 1 minute each time -- without getting grease on your hands or clothes." (On the Dynamic website, they talk about Gen. 3 vs Gen. systems, with the Gen. 4 requiring less maintenance.)

"The Dynamic Shaft Drive System is a unique “chainless” drive system..."- from the Sussex website

"Our shaft drive system uses CNC machined spiral bevel gears made from hardened chromoly, as well as sealed cartridge bearings, and cast aluminum housings. The shaft drive system was desinged to last 6,000-8,000 miles; however, this range can vary greatly depending on how the bicycle is ridden. More aggressive and heavier riders, and riders on hilly terrain will get a shorter life from the gears and bearings; more recreational riders on rolling terrain will get a longer life. "- from the Dynamic website.