Pros : I've tried several different consistencies, and it seems to work best when really thick.
Cons : It can flake right off of the glass when you are trying to carve it and clean it up, but I've found that I can put a little more wet medium down and sort of 'glue' the piece that fell off back on to the glass.
Other Thoughts : It would be nice if it would mix to a consistency that was really smooth - i expected it to draw on the glass more like those 'puff paints' or glitter glue does, but its not that smooth. I recommend making it super thick and using as small of a hole in the bottle cap as possible while still getting it to flow out. Cleanup and carving with various sizes of xacto knives works best. Also, rather than carving pieces away to clean the edges, you can push the edges inward to make them sharper. That helps prevent too large of a chunk from getting cut away by accident. all in all, a fun new way to expand your creativity!!