Although parts of the store are cramped and dark, and I have yet to purchase anything Prada, it doesn't stop me from loving the stairs in their Soho store (designed by Rem Koolhaas in 2001). I love the alternating concrete and wood treads. This part of the space is nice and light and open.
If you want to see a virtual walk through of the store there is one on Architectural Record's site.
(photos: sinorfavela/ / CC BY-NC 2.0)