How much of a factor are they really, timing chain tensioning notwithstanding. I've noted Bushnell ( http://www.bushnelltandems.com/ ) & Phil Wood ( http://www.philwood.com/products/bbpages/eccentric.php ) both market them, as does (to a much lesser extent) Co-Motion. Is one measurably better than another, or is it marketing hype? Part 2 of the equation: would be a better pick over the others with regard to fitting the new R601 captain's crankset?
I do not know if it is the best or not but Tandemgeek used the Bushnell on his Calfee build and when we talked to Calfee and told them that we would probably eventually upgrade to the Gates belt drive they recommended the Bushnell. That is what we have on our Calfee Tetra tandem and it works perfectly with the R601 crankset.