I'll just chime in here and remind every one that, averagely speaking, advantage translates to a +5 bonus, even if it doesn't stack.
Most sources I have seen actually say Advantage/Disadvantage is roughly +/- 3.3. I haven't done the math, but just pointing out what I have seen
This is true in a vacuum, averaging over all possible DCs. Advantage is equivalent to +5 when you need an 11 on the die, +3.75 when you need a 16, and +2 when you need an 19 on the die (symmetric for die rolls lower than 11). All this averages out to +3.3.
Given that 5e has bounded accuracy, most checks require you to roll somewhere between a 7 and 14. These are regions where Advantage is closer to a +5. Then you have to take into account the fact that (for attack rolls), advantage makes it twice as likely you'll roll a 20 (for a critical hit, auto hit and 2x damage) and disadvantage makes it twice as likely you'll roll a natural 1 (auto-miss). Taken all together, Advantage gives closer to a +4.2 bonus.