Pros : With Rubyfulid flux, the solder always sticks to the copper foil and not the glass, leaving no mess and a quick, easy, smooth solder seam.
Cons : You are advised to keep it off of your skin, and wash your hands well after using--but the Pros far outweigh the Cons.
Other Thoughts : I tried other flux but it made a mess of the solder and took too much of my time to clean it up and make it look good. I switched back to Ruby in a hurry.