We tried a Rohloff on the tandem for several rides a couple of years ago (about 500 km). It didn't shift anywhere near as well when under load as a derailleur system does. We like to have our cranks 90 degrees out of phase to make the power input smoother when climbing, so there is no dead-spot in our pedaling cycle in which shifting on the Rohloff would be easier. I therefore had to ask the stoker to ease off every time we wanted to shift while going uphill, which I don't need to do with our derailleur setup (I only need to do this for front shifts), doing so was not only annoying but was causing us to lose quite a bit of momentum every time.
In addition, getting tighter spacing of the gears is the main reason for wanting to switch to an 11-speed cassette; using a Rohloff would take us in the opposite direction because the gear spacing on the Rohloff is not at all ideal for road use. The 13.6% difference between each gear is right at the edge of what I'm comfortable with on a road bike (tandem or single). I can tolerate having one or two jumps like that in a cassette, but prefer most to be in the more ideal 10-12% range. A road version of the Rohloff hub would have 12% differences between the 14 gears, giving a range of 435% (the 526% range on the Rohloff is unnecessary on the road).
Despite all of these criticisms of using a Rohloff on a road tandem, I'm certainly not a Rohloff hater. Having taken the Rohloff off of the tandem, I put it on a hardtail 29er MTB that I wanted to build for myself. For that purpose (a single-bike MTB), the Rohloff is the best transmission available - I love the reliability and (external) simplicity. The gearing range and spacing is very appropriate for a MTB, and I can make it shift very smoothly by timing the shifts to coincide with the dead spot of my pedaling or I can ease off slightly if needed. Once my wife road my MTB, she wanted a similar setup, so we now have two hardtail 29ers with Rohloffs (they are our only MTBs, we are roadies at heart) and both love them. We'd never consider putting a Rohloff on the tandem again, but I'm glad that we tried it so that we know the pro's, con's, and limitations. YMMV.
Last edited by Chris_W; 12-23-14 at 05:02 PM.