Urbanisation in the 21st Century and its future impact

Recently, the percentage of the world's population living in cities just passed the 50% mark.

How does this impact on our lives and is the increasing proportion of people living in cities likely to be positive or negative in the future?

I'd offer my own answer, but freakonomics blog has already asked several thinkers on this point and they've given a vast array of opinions. Some think negative of the trend, I'm more inclined to sympathise with the most positive point of view put forward by Glaeser.