Lots of misinformation in the discussion above. I’ve actually been riding a Pinion-box MTB for nearly a year now, using the 12-gear version and belt drive.
Facts:
1. There is a weight penalty of about 1kg netting the gearbox and belts/singlespeed hub against a derailleur and chain set-up.
2. The system has been near-100% maintenance-free through a year of British weather and about 3-4 rides per week; just a hose-down after each ride and a little spray-lube around sensitive points (probably not even needed).
3. There’s a theoretical efficiency deficit of about 2-3% compared to derailleur. In practice, this is more than offset in offroad use by the ability to switch to any other gear near-instantaneously and the lack of crashing changes and chain-drops. I routinely overtake co-riders at the bottom of sudden hills whilst they mash through their gears and curse…
4. The need for a (really minuscule) back-off from pedalling to make the gear-change is in practice just not an issue. It takes a couple of rides to adjust, but is fully offset by being able to change when the bike is stationary.
5. The use of a gripshift rather than triggers seems to excite naysayers; in reality it just isn’t an issue after you get used to it. In the first couple of weeks I made one or two unintended changes, but that never happens now I’m used to it.
The main downside so far is that take-up of the system by MTB makes outside Germany has been slow, but the choice is growing if you’re happy with hardtail (I am). So far, full-sus implementations have been slow to take off (basically Nicolai and Zerode). With the new lighter and cheaper C-line Pinion boxes arriving later this year, I think that could rapidly change.