Chelsea's is a warehouse of vintage clothing. If you ask for a black dress, they will take you to a row, three high of thousands and thousands of black dresses. It is mind-boggling, and you could easily get a whiplash as your head shoots around trying to take everything in.
Definitely the place to go if you want something unique for a costume party. Tons of clothes, coats and accessories -- and "tons" is understated. Sometimes it is overwhelming, so leave plenty of time to dig through the treasures.
The only reason I won't give this a 5 is because of the dust. Everytime I go in there my allergies act up. Leave a bread crumb trail on your way in so that you can find your way out.