Made this for the third or fourth time now. Things I've learned:
Shred the ginger even if it's a royal pain in the ass. So much juicier that way, so much more flavor. Chopping doesn't get a tenth of the flavor that shredding does, seriously.
As said by others: fresh ginger for the win, don't use old stuff it won't give you good results and will just be sugar water.
This round I've done 3 and a bit cups of shredded ginger (with their juices), it was pretty soggy so may be more if measuring 'loosely packed' style. And 3 cups of sugar. I think one could go down to 2 cups and be fine as well, if your ginger is good and strong.
3 cups of sugar it's still a bit more than some soft drinks for sugar content per volume, though less than some as well. Makes me feel better about having it if it's lower in sugar, for sure. 4 cups was too sweet for my liking.
This current batch is chilling and getting gassed right now, my last batch I bottled after kegging and found it had started to ferment (not sure how, something must have gotten in) so close to bottle bombs....fountains of foam! Be thee ware!