I have bought a pleated skirt, cowl-neck knit fall/winter dress, and lined cream blazer from Mango over the last year. I didn't really like the blazer because it wasn't white, it was cream. Returning the blazer was easy. There is a code that the lady at Canada Post used for the return to be free (shipped back to Richmond Hill). It took maybe another week or so for the refund to appear back on my credit card.
I like that the prices you see are the prices that you're charged (no customs, duties or exchange rates).
Regarding the sizing, I found that it is true to size for me (medium for the skirt (waist is about 30"); large for the dress (to accommodate my broad shoulders); size 8 for the blazer (again to accommodate my broad shoulders).
As others have mentioned, the quality was comparable to Zara. I was a bit disappointed with the pleated skirt, as in the pictures, the pleats look deep, but in reality the skirt is more flowy. I kept it because I like the colours. It was weird to see that The Bay actually had the same skirt after I had already ordered it, but it didn't matter because they were the same price.
To shop at Zara or Mango, I have a shopping list and filter my searches. For example, dresses in red; blazers in white. At Zara, occasionally I may find something in the store, but generally I find the experience and selection overwhelming and chaotic. I ship my purchase to the store (don't stand in the regular line up to pick up your package) and depending on the length of the line, try it on at the store, or take it home. Returning at the Zara store is easy too.