Pillowball top mounts are designed with performance handling in mind. OEM are rubber mounted bearings (if any) and this rubber flexes quite a bit. This is great for reducing noise and vibration, but not so good for feedback+handling purposes.
Pillowball mounts use an aluminium plate (usually) as the top damper mounting, with a spherical bearing used as the means to keep it attached flexibly yet rigidly to the top mount.
Good: Better feedback
Bad: More NVH (Noise/Vibration)
The last set of D2 coilovers we put on came with Tein Pillowball mounts recommended by D2, and didn’t even fit the dampener rod (incorrect diameter) and were unusable. (Get a matching suspension!)