The issue is one of buy-in. For it to work, everyone involved, client, management, development, qa, design, everyone has to be on board with the agile way of doing things. Without that, you have development working in the agile manner, believing in good faith that everyone else is too, and then the second a snag gets hit, then that all goes out the window, and development is usually the one left holding the bag and punished with overtime.