Maybe not entirely relevant, but I worked for 5 years before going back to school for a Ph.D. (am just starting now). My job was aerospace engineering in the US, and what I found is that if you want money or managment positions, then the PhD is unnecessary, but if you want to do research at your company, it's very difficult to convince the company to keep work in house when they can farm out to a PhD, (either a consultant or Univ).