aliEnRIK:
I have various ones. The best ive ever bought was a balanced mains transformer. Theres no cons to one except the sheer size and it buzzes in use
As for actual mains conditioners ~ Most work well with sources and low powered equipment. many have found amplifiers (Especially power hungry ones) can sound worse as the conditioner is robbing it of power (tacimas are favourites for this problem)
Personally I use an Isotek Sigmas so it doesnt rob my amp of power
Pros ~
Assuming no loss of power then everything should sound better, more natural. Pictures on the tv will look more detailed and natural
All this said, if you have perfect mains to begin with then youll reap no benefits
I am currently using a Russ andrews platinum mains filter and I am not sure if it is robbing the power of the amp...like you mentioned. I am now currently playing the same track Peter Gabriel ''secret world'' over and over again, with and without the mains filter...it seems to my ears, with mains- the sound is cleaned up a bit but kind of lost the excitement, space and intensity of Peter's voice (if that makes sense). I have no idea which is best at the moment....