I used a rain skirt for a few years now, I started to use them because I really dislike waterproof trousers. I bought my first one, then lost it and after that, I had them made by my wife's friend, I used Silnylon and then a couple of heavier-weight materials. My latest one is from Amazon, cost me £10 and in some ways is the best one, the design is not just a rectangle of material, its shaped, less at the top more at the hem. The only problem for me is the colour, pale blue!
Like all things it has a disadvantage, it can blow around easily in the wind so my knees were often wet, but once I fitted weights (bought from Hobbycraft, ladies sew them in to keep skirts from blowing up) the problem is more or less solved. I use the same type of weights around the hem of my poncho.
The best things about a rain skirt are (as already stated) it can be put on and taken off in an instant, you don't have to stop walking either, it weighs next to nothing and is tiny in volume to pack in a rucksack or to keep in one's pocket.
Definitely, one of my better buys!