Actually it is more than you would think. The longer the cable, like on a tandem, gives the derailleur or shifters more "space" to compensate, if you think about it logically.
I have been using a jtek shiftmate for a couple of months and I love it! But I am using campy 10 speed with shimano 9 so I do need it. But I would still buy it if I were using all 10 speed and just mixing brands.
The only thing is that you need to give it a couple rides to break in (the cable that is). Then the cable stretches like normal, and you do a little adjustment.