My closet contains designer pieces as well as cheaper clothing. Among Mango, Zara and H&M I absolutely love H&M, especially the more expensive lines. I feel that quality is much better, at a better price. I don't know if things are different at the other side of the Pond though. I love H&M because designs are a little bit more daring than Zara and Mango.
H&M comes from Sweden, I have the impression that their designers do not only make copycats of the high end fashion labels, but have also interesting and original ideas. I have several items that seem much more costly than what I paid for them. Last week I purchased a formal black jacket styled as a kimono, really very nice, for 39 Euro (+/- 50$). I have also to note that in Italy we have either unstyledm crappy clothing stores or very high end stores. The only mid range stores so far were Benetton, Sisley and Stefanel, but I have always found their designs extremely boring.
Zara and Mango (the latter a little more expensive) are both spanish, their size system is similar and their sizes are in fact smaller than the average. I'm an Italian 44, a denim 30, a UK 14, a EU 40 and a US 10. I can buy H&M clothes everywhere in the world without trying them on, but I have to try every single piece on in Zaras and Mangos, and sometimes I need to go up even two sizes! I am tall (5'10) but not fat (140lbs) and sometimes I do not fit into an XL! I would say that Zara and Mango are ok with people with no hips. I have quite some rear bumpers so maybe that's the problem!
Zara and Mango stores are also always filthy here: dusty and chaotic, and the sales persons are always acting like they were doing me a favor. Oh, please. What an attitude!
Overall, I have several Zara and Mango pieces, but I buy them only if they are really really cheap, no more than 30$. Over that, they are overpriced and not well cut. The pants, for example, are sooo unflattering! I prefer H&M 100 times more! And everywhere in the world I go, H&M sales persons are always nice, not like those others I have mentioned.