They’re great for sitting around in campsites and such like. I only use mine for winter climbing for which it is superb, I prefer a conventional layering system for everything else. I walk to the crag in a base layer then change for a dry base layer t-shirt and don the Montane. Even in really shitty weather I don’t feel the need for a hardshell. I would overheat walking in it on all but the nastiest of days.
Main advantages are low cost, high level of weather protection from a single garment and the fact that they are still warm when wet.
Disadvantages are weight, large packed size and a lack of the versatility you get with a conventional layering system. And of course it isn’t waterproof.
They don’t work for everyone but if they work for you they’re a top bit of kit.