I am a relatively experienced DM who felt comfortable going into SKT, but I'm certain Sean McGovern's guide helped me give my players a better experience than I would have been able to without it.
For a new DM, this guide is invaluable-- sections of the adventure (especially Chapter 3) can be overwhelming, but this guide streamlines it and works like a Cliffnotes that is easy to refer to before diving into each chapter for more details.
As an experienced DM, what I found most helpful were Sean's comments about what elements he felt needed foreshadowing-- what aspects of the plot needed more clarity. I basically used his list and worked those into the early sections of the story based on my players' choices. He also inspired what was one of the most memorable fights of the campaign during the Berg of the Frost Giants chapter (don't want to give it away, but it was an awesome set-up the players loved). Finally, the background information he collected on Iymrith helped to make her a more memorable foe-- not just a standard ancient blue dragon.
Overall, Storm King's Thunder is a flexible story-driven campaign. Sean's Guide to Storm King's Thunder is especially helpful in streamlining the aspects of the story that MUST happen to drive the story forward, calling attention to how the DM can provide a better plot flow, and inspiring creative use of the various villains and settings throughout the adventure.