As a power user, Hydrogen Pack is one of the first plugins I install when working on any Oxygen site. The customizable keyboard shortcuts, improved drag and drop in the Structure Panel and being able to copy and paste any element are must-have features in my opinion for any user looking to work faster and better in Oxygen.
Hydrogen is simply the best thing that happened to Oxygen since Oxygen exists! The Keyboard Shortcuts are incredibly convenient, and the copy/paste feature makes me save a lot of time. I just can't use Oxygen without Hydrogen pack now.
I rarely buy plugins, and I like to solve tasks on my own, without depending on additional plugins. Hydrogen is one of those that I bought because it really speeds up the workflow. Now I use it on every project.