The faced nav works fine if I just use that. I can filter by gender, brand, category, size, colour and price to narrow down my search. All the things you'd expect a fashion retailer's site to do. (Urban Oufitters, I'm looking at you right now - why on earth do you think I wouldn't want to narrow my search for an item of clothing by something as important as colour or brand? Faceted nav just too uncool for you, is it? Having a site that looks like it was built in the 1990s is perfectly on-brand for your retro styling, is it?)
Anyway, back to MandM. If I start my journey by using their top menu, my choice is not always passed across to the faceted nav. Some options take me to a custom landing page for my choice but faceted nav is not updated. Where there is no custom landing page and I'm given a set of search results, the choice is passed across. I can see this being a cause of frustration. I've clicked on Mens in the top menu, so you should be able to update Gender in the faceted nav for me.
Another frustration is around sizes. See the image below:
Or maybe I should just be happy that I can actually narrow my search by size (Urban Outfitters, I'm back onto you).