Barry Boehm sort of summarised this fairly well in Balancing Agile and Discipline, although I'm sure he would disagree with my characterisation of it:

- If you have unpredictably changing requirements, and a small-ish non-life-critical product to build, and a large fraction of good people on the team, you should use an agile process.

- If you have unchanging or predictably changing requirements, and a large fraction of not-so good people on the team, you should use a plan-driven process.

But what if you have unpredictably changing requirements and a large fraction of not-so good people on the team? No process can save you now.