Assuming that the pure waterfall model is always a bad idea, when do you use an adapted one as an iterative or spiral model and when an agile model?
Here’s a rule of thumb: use waterfall for projects where there isn’t any invention and/or where the problem domain is well-understood, e.g., building a house, installing a server.
Use Agile for projects where the problem domain is not well-understood, e.g., creating a new software product, rewriting an existing system using a new environment (mainframe app to client-server).
A well-understood problem domain is one where the changes during the project are less than 1% of the scope.
My preference for agile projects is to use Scrum for all but the simplest projects, or Lean for cyclic projects like sustained engineering.
And, regardless of the methodology, you still have to do it right. No silver bullets here.