Overall, I can easily and wholeheartedly recommend the Corepad Skatez to anyone looking for a better, smoother and longer-lasting glide for their mice. One of the manufacturer's best features is that they come out with these replacement feet very quickly after a new mouse is released (this applies to bigger brands mainly, but they offer Skatez for some mice from smaller companies as well).
Even though I have no proper tools for measuring friction, I hope my makeshift testing method could illustrate the difference between a stock and Corepad feet. However, in my opinion, one needs to experience this in person to truly be convinced.
Usually, the pack of two replacement feet costs €7.99 (about $9.30), which is a fair price considering the competition. However, in my experience, Hyperglide and Hotline Games mouse feet tend to break in faster. As for their actual durability, it's really hard to say which one does "worth the price" better.