I forgot to add this to my last post so I've decided to do it now. I got a Rimmel London nail polish in Disco Ball in my stocking on Christmas day and here is the review for it. The polish itself has got silver and blue sparkles in a clear varnish.
From inside the bottle, the polish looks like it's packed with glitter, but in fact, when you apply only one coat the glitter is very sparse. In my next picture, I've layered the nail polish 5 times on each nail because I didn't put a coloured nail polish under neath it. Of course, if you put another colour as an undercoat, you could put only one layer of this polish.
As you can see, it almost looks like a glittery silver nail varnish rather than just an over coat.
I would say that one disadvantage of it is that it's really easy to peel off if you put too many overcoats.
Overall, I would give this polish a 4/5.
(losing one mark because of the sparse glitter and gel-like quality it gets when there are lots of layers)