| OnlineDM |
An important point to note from my blog post about advantage/disadvantage being roughly equivalent to +/- 4 or 5 is that, in D&D the number you need to roll on the die in order to succeed is usually somewhere in the 7-14 range. It's rare that you're in a situation where you succeed even if you only roll a 4 or 5, for instance, and it's rare that you're in a situation where you need to roll a 16 or 17. Those situations tend to be trivial or frustrating, respectively, and most DMs don't put the players in them very often.
So, GIVEN that you need something between a 7 and a 14 on the die, the impact of advantage or disadvantage is similar to the impact of having a bonus or penalty of 4-5 to a single d20 roll.
If we're looking at the extremes (chance of success/failure when you only need a 2 or better, or when you need a crit or nothing), then the impact of advantage is much smaller. But those situations tend to be rare in real games of D&D.