Cuts off the feedback in live situations well, but I can't help but to notice the darkness they bring by cutting the highs. Good if your vocalist either a) signs quietly very close to the mic b) shout loud a bit further away. In both cases when the feedback is not that critical you can EQ highs a bit and you are good to go. However, if the singer does not have the control of the mic by for example holding a guitar the situation is more difficult.
I wouldn't use this in studio, except for snare: then the darkness is actually quite nice when you still gets the bright snaps from overheads.