Downtowns, by their nature, are the very epitome of mixed use. They are almost always populated (to some extent), sport shops and restaurants, as well as providing employment. It doesn't get more "mixed use" than that.
Public transit systems are at their most efficient in these environments. In my experience the cities that offer the best mixed use neighborhoods have the best transit systems, if for no other reason than high density, mixed use areas provide the highest economic impetus for transit in the first place.
How many downtowns are livable and enjoyable after 6pm? Some have pockets of activity, usually built with lots of subsidies and great effort (and they often have an eery feel of the-city-as-imagined-by-suburbanites). Skyscrapers are density without any of the advantages of density (http://newurbannetwork.com/news-opinion/blogs/michael-mehaff...). Even Manhattan, a kind of endless downtown, is at its best in the somewhat lower density areas that have fewer skyscrapers.
Downtowns, by their nature, are the very epitome of mixed use. They are almost always populated (to some extent), sport shops and restaurants, as well as providing employment. It doesn't get more "mixed use" than that.
Public transit systems are at their most efficient in these environments. In my experience the cities that offer the best mixed use neighborhoods have the best transit systems, if for no other reason than high density, mixed use areas provide the highest economic impetus for transit in the first place.