For reference I'm 5'7", 130lb and I purchased the size 5. If you are looking for the same fit as what is shown on the model,  then go by the size chart. If you want something that fits a little looser, then size up.  I'm going to return mine for the next size up. I'm not comfortable in skin tight pink pants!
The quality was fine. The weight and feel of the fabric was fine. So no complaints there.
I'm giving it  5 stars for the accuracy of the size chart and the quality of the pants for the price.