Re: speed ball jump ropes...worth it?
FYI, Molly Metz ^^^ did something like 1,400 double unders in a row. I've met her personally and would have bought some speedballs that day if I hadn't already spent all my money on other gear!
Also, I picked up the JumpNRope rope progression (beaded, Trainer, and R1 ropes) and in 7 weeks of using the beaded and trainer ropes, my mad effort unbroken double unders went from 62 to 190! Definitely worth it. The heavier rope is easier to "feel" and helps immensely to develop strength / strength endurance in the shoulder musculature.