As I received a PDF copy I cannot comment on the quality of paper used, but I can tell you that each design is single sided (nothing printed on the back) so if you wanted to use Sharpies or alcohol-based pens, you don't have to worry about them bleeding through and messing up the illustration behind.
Line thicknesses on the designs vary from thin to very thick. Personally I prefer thin, delicate designs but a variety of both is always good in a colouring book.
Here are some pages that I coloured, using my triplus fineliners, twistables and assorted gel pens. Hope you like them!