In addition to oozing atmosphere in the manner that any story that at all involves a tailor properly should...
...these are all quite compelling characters. Their relationship, too, feels quite natural and unforced, and furthermore the issues they have and the ways they resolve them seem quite appropriately tailor-made to fit the characters involved, rather than just grabbed from the bag of cliches.
I wouldn't say that it feels like a masterpiece. Rather, it feels like fine craftsmanship; everything fits together neatly and snugly and purposefully, but whilst never neglecting aesthetics.
Also tailors are just so fucking cool. (Though honestly that part of the premise seems all but forgotten in the second half of the manga, albeit not to its detriment. But it definitely influenced my opinion a little anyway.)