I just started Ear Training yesterday and I’m now on the first lesson of Grade 1 - The prefect 5th.
I sorta had an epiphany in that not only can I now pretty much put my finger anywhere on the fretboard (if my fretboard memorization kicks in I may even know the root note) and can now find the perfect 5th. But as an added surprise, I can see the relationship to the entire scale.
That’s sort of a cool, light-bulb goes off in my head moment. 
Up to this point I’ve used notes to learn scales, but this is The First Time that I have seen the scale as an interval. That’s why, for me personally, this is sort of an epic “Ah-Hah!” moment.
