I've been experimenting with Citadel metallics and painting some bits (as well as finishing up the piece I last worked on), and it's interestingly different. From what I've been able to gather, most metallic paints get the shine from ground mica flakes--there are genuine metal metallics, but they're more finicky in that the metal flakes will rust in contact with water, so you have to thin with nearly pure alcohol, clean brushes used with alcohol-based cleaners, etc. The Citadel line, from what I've read, uses a finer ground mica than other lines but is otherwise a water-based acrylic. These qualities give the dry paint a smoother, thinner finish. So far, so easy.
The tricky part with the Citadel metallics line--especially the gold--is that the coverage is weak in comparison to others I'm familiar with. To thoroughly cover a basecoat would require 3 or 4 layers of the unthinned paint, and even then the underlying color might come through. On the other hand, this color can be used to my advantage--subtle shifts of hue can be had just by basecoating the area with a different color, and the metallic effect is strong even when the basecoat is still coming through. I've found a couple of basecoat mixes I quite like for various shades of gold, and the end results are nicely distinct to my eyes. Citadel discontinued one dark copper entirely which I use pretty heavily for one scheme in its Vallejo formulation, and I'm interested to see how well I can mimic it with a dark undercoat and the light copper.
I'd actually be interested to know how the rest of the Citadel line has improved in its latest incarnation. The pots alone are a vast improvement over their previous version, which was probably the worst possible choice in every way--hard plastic with a screwtop lid is difficult to the point where I had to use a wrench to open a used pot, yet simultaneously more air gets in, giving the paint a much shorter lifetime. I've got decade old paints that are still fluid, but the last Citadel paints dried up after a year.