I acquired my knowledge and skills in ISD from the creators, which was the United States Air Force, during the second-half of my 20-year career. It consisted of 6 weeks of classroom training on Instructional Design and Teaching methods.
After retiring in 1999 and moving into the civilian sector, I quickly realized that following the ISD process to a tee was not going to work. My course development had to be much faster, as it always seemed that courses needed to be developed "yesterday"!
With the help of my leadership, I learned the basic tenets behind Project Management and Six Sigma, and incorporated the best of all three methodologies into my work. This led to a very flexible, yet standardized approach to Instructional Design. The major change to my approach in ID is in the Analysis phase. By looking for key requirements up front, I can quickly determine the appropriate approach.
For example, if I am developing a certification course, then I primarily follow ADDIE. If the course requires tests and a passing grade to receive credit, then a combination of ADDIE and PMP is best. If the course just provides proof of attendance and no testing, then ADDIE and Six Sigma prove most effective.