Works great on both electric and acoustic strings. Clears up the sound, and makes the strings feel more responsive. This is probably simply because after cleaning the strings (with anything) they become less sticky/grimy, so you could probably get that effect however you clean your strings, it doesn't have to be GHS Fast Fret...
But here's the thing, there probably isn't a more convenient way to clean your strings than GHS Fast Fret. Keep it in your gigbag and it's always there when you need it. No spills, no hassle, just a quick wipe of the strings and your done. And the box is large enough so you don't lose it and small enough that it fits nicely everywhere.
The only complaint I have are the razor-sharp edges of the metal box. And I'm not kidding, the edge really is sharp enough to cut a really nasty wound in your finger ruining your playing for weeks. Absolutely terrible design here. The boxes used to be plastic, and not have this problem. I can't think of any good reason why they switched to razor sharp metal instead. Extremely poor decision! So one star comes off because of that.
So in conclusion : Superb product, but with one flaw that might end up ruining your gig or recording session if your distracted while opening the product.