Zara is okay. I used to own several of their shirts but they definitely start to fall apart at the seams, buttons fall off, etc. after several wearings. I've never looked at Mexx?
In my opinion, Zara is best for their jackets/coats if you're planning on wearing it for only 1 season. You can grab them for ~ $150 and they are cut much better than what you would get at BR/J Crew and they don't break the bank.
Costume National is a great brand, but as mentioned above, very expensive. I own several coats from them and will have to admit that even though they look great and are cut slim, the quality is lacking for the pricepoint.