This thing performs a lot different than just a round half-sphere would. The corners add a little bit of purchase at the cost of comfort but you're also a lot less likely to slip off this thing than if it didn't have corners. I also have the 5in one and I like it a lot more but honestly that's just because this one is a lot harder. It's on a 30 degree wall and its pretty tough to do much with without good feet and another hand. Desperate fun!