The Waterfall method is great for establishing the initial scope and functional specification document (including the wireframes, IA and expected UI/UX).

From there the Agile method can be used to tweak things throughout the project.

I think it’s valuable to include “Change Requests” though even as part of the Agile method, as every iteration of a change is going to have an unknown affect somewhere else, and often hemorrhages time and money from the project.

The BEST project methodology is a little something I like to call “Common Sense” Project Management. Use the tools you need to get the results everyone wants.

A combination of methods is also required when providing paid SW delivery services to customers, since most customers want to know what they’re going to get for their money.

For many first-time development relationships where trust is low, a firm fixed price tied to a detailed scope of work is most typically required. This is equivalent to a Waterfall method.

Other relationships are based on a time and materials basis. That is, the customer pays for whatever it costs to make what they want. This lends itself to Agile methods.

While applying Waterfall to time & materials contracts will work (think defense industry cost-plus models), trying to apply Agile to a Firm Fixed Price contract will more often than not result in running out of budget before the system is complete.