Me niether, but I would ask why bother using it? I have a tandem with Campy 10 shifters and a Dura Ace RD(9spd), no Jtek and perfect shifting... if you don't have problems on a tandem with a 9' long shift cable, I can't see how the thing is necessary.
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.
I can go both ways mentally with it I guess. I will have to stew on this one for a bit.
To be fair I do not have personal experience with it on a single, but my freind with the cyclocross setup says it shifts fine. I guess better safe than sorry for the small amount of coin the Jtek is... of course why not try without first then spend the cash if you don't like it.