While these things may be OK for players with shorter, 61 note keyboards, I would advise anyone with a 76 or bigger to take a p(ass)...

There's a reason pianos have BENCHES, not stools... To get good hand position at the extremes of the keyboard, especially when both hands are at one end, it is necessary to 'scoot' over just a tad, and these types of stools make it impossible. The twisting and bending of your torso and arms don't make for much comfort during this time, and lead to improper technique and strain.

Kind of like the contortions some of us make trying to turn a little ribbing into a full blown conflict...