This store makes 1.5-2 million a day. They'll make it back within a week, the press coverage alone is worth it.
The publicity alone is worth more than the investment.
From a business point of view, the investment is trivial compared to the advertising an PR return.
As for the cube - it seems almost to disappear leaving the Apple logo the center of attention, like it is magically hanging in space. I like the design, for what that's worth.
As for the criticism that the money could have been used elsewhere for humanitarian purposes - this could be argued for any non-humanitarian use of any corporation's funds. I am in sympathy with the position - there is never enough money spent to feed children, research diseases, help others. But at least for Apple, the $6.6 million dollars represents a trivial amount. They could have found that much between the cushions of the corporate couch. If Apple was going to contribute to humanitarian causes, the $80 billion in cash reserves provides more than adequate funds to contribute - if that is Apple's desire. Whether Apple wishes to make such contributions is fodder for another thread.