I've got modified tuners, preamps, phono stages, and power amps and, as a generalization, Black Gates almost always sound better, when used correctly, than the equivalent electrolytic capacitor. Of course I'm a little nuts -- look at my system. It has a power amp with 160,000 uF Black Gate cap supply (one of the signature Krells supposedly has them too but I've never seen one).
They are good as power supply caps and can be excellent in coupling and by-pass applications. They will fit where larger teflon film caps will not. Note that there are several variants of BGs --Standard,FKs, Ns etc. and these sound quite a bit different depending on how and where used.
Some people do not like their sound. Others don't care for a particular type. It is possible to use too many, but this is true if you use too much of anything. It's also possible that a circuit might end up sounding worse but that's usually a problem somewhere else.
It's not an accident that they're popular with many modifiers and diy's. They will be missed!