Constructed by building tycoon Minoru Mori, the mega-complex incorporates office space, apartments, shops, restaurants, cafés, movie theaters, a museum, a hotel, a major TV studio, an outdoor amphitheater, and a few parks. The centerpiece is the 54-story eponymous Mori Tower. Mori’s stated vision was to build an integrated development where high-rise inner-urban communities allow people to live, work, play, and shop in proximity to eliminate commuting time. He argued this would increase leisure time, quality of life, and benefit Japan’s national competitiveness. Seventeen years in the making, the complex opened to the public on April 23, 2003. source : wikipedia
I was there during Jan 2007. It is an up-market shopping area with lots of designer shops and restaurants. Up on the observation deck of the Mori Tower, you can get a spectacular view of Tokyo city.