I use Intellij IDEA for Java programming, but have not really considered it for Mathematica. I saw a video with a Wolfram programmer who was describing it as "essential" for him, but from what I could gather of what he said, most of the benefits for him were either workgroup related (sharing code with other Wolfram programmers) or were doing things involving "internal" Mathematica code, not notebook code/problem solving.
So, for a person doing normal Mathematica calculations and models in the science world, what, if any, are the significant benefits to using Intellij IDEA?