It does indeed reduce the sound a lot! The compromise to the sound quality is not that big because you can play in an appartmet with an acoustic set without having to spend great amounds of money to a good e-kit. I use those with the Millenium QuiHeads and I have to admit that they sound a bit more compared to the toms/snare/kick drum, but it can be adjusted by either puting a piece of cloth on top of them or adding a gaffer tape below and at the top of the quiet drum heads to increase their volume.
Eitherway, my experience with them so far is good and I totally recommend for the ones that want to use in an appartment.