Believe it or not, I've seen waterfall work, I've seen waterfall fail. I've seen agile work and unfortunately I've seen agile fail more often as people who don't understand what they're supposed to do create a clusterfuck of a process that just gets in the way. there was an article on HN not too long ago by an old school developer about development methodologies and how he didn't really think it made a difference. What he saw made a difference was how well the team communicated. People over processes, the very discussion of "Agile" ad nauseam violates that very rule. For crying out loud the process will not save you! try to find a process that will not get in people's way while allowing people to communicate effectively. I'm sure that some social scientist might have a better idea on how to solve it than another random developer who thinks he's God's gift to mankind.
Not in my experience. If there are a bunch of PMs and managers, you're going to find that they keep right on doing management and PM stuff, no matter what Agile Manifesto they think they latched onto.
Management: "This project is going to take you 13 sprints."
Right away, you're in trouble.
Management: "What's with your burn-down rate? We're four days into this sprint and we don't see any progress."
What replaces management is micromanagement.
Manager in the middle of the daily scrum: "Okay, let's go around and get status from everyone."
Yup. Status. For management.
Management: "You need to drop what you're doing and work on feature X for next week."
"But we're in the middle of the sprint."
To make it really work, you have to start nuking job positions. And at a big company that is hard.
Bingo. You know the MacLeod Pyramid (Losers, Clueless, and Sociopaths)? Most of "Agile" methodology is Revenge of the Clueless. It's a great way to replace a company with one long meeting that never ends.