The Nittaku Premium 40+ is a huge improvement over the previous XSF and DHS balls. This ball feels heavier and more solid compared to to the previously mentioned ones. The speed and spin is also superior. Haven't had any roundness issues until now. It also doesn't have the bothering sound of the XSF balls. The only minus is that NP40+ is more expensive than the XSF and the DHS ones.
Edit 7/2016 : These balls are very consistent and have a predictable bounce. They can last even longer than cell balls if you don't hit many edges. If you hit the ball with the edge quite often, it can break very-very fast, this being the biggest vulnerability of all plastic balls. Even if the XSF balls seem more durable, the NP are definitely more pleasing to play with.
Edit 7/2016 : These balls are very consistent and have a predictable bounce. They can last even longer than cell balls if you don't hit many edges. If you hit the ball with the edge quite often, it can break very-very fast, this being the biggest vulnerability of all plastic balls. Even if the XSF balls seem more durable, the NP are definitely more pleasing to play with.