Yaly is a good place to go for items that require attention to detail: apparel with sequins, ruffles, multiple layers of fabric, etc.  They do a decent job of replicating designer dresses, blouses, and pants, but make sure to bring them as many photos as you can of the desired item. And before you run down there with your Paris Vogue in hand,  you should bear in mind that though they certainly have the most variety in Hoi An as far as fabric is concerned, they don't have the quality or quantity of fabric available in the Western world.  If you want the most for your money, purchase the material you like and bring it to Vietnam.  And, just like any other tailored clothing shop, Yaly will need you to make multiple visits for adjustments, so make sure you set aside at least 3 or 4 days in Hoi An.

Now if you're looking to buy run of the mill suits (flat front pants, pencil skirts, and matching jackets in all the most mundane and conservative fabrics), you can go to any of the other 400+ shops to have them made for significantly less.