I got the Elite Fenestrated Sheepskin, and my immediate reaction was how much more comfortable they are than the stock Hifiman pads. I have long hair, and the stock pads were slippery on hair causing the headphone to slip forward over time. But, these are gripper and don't have that issue. Additionally, the clamp is better distributed because of the memory foam which is better for glasses users.
Sound is slightly better, marginal improvement.
I love velour earpads and just picked up a pair of the Edition XS headphones from Hifiman. Read great reviews about these pads and immediately purchased. They've been fantastic! Sound is very close to stock, and comfort is improved. Well worth the purchase for people who enjoy velour earpads!
In my opinion these pads are worth it for comfort alone. An added bonus is they reduce a lot of the listening fatigue caused by treble peaks. After EQing a small bass shelf I find my headphones much more comfortable and pleasant to listen to.