I didn't enjoy advantage for flanking much for our group. It makes melee characters more powerful, since they were almost always attacking with advantage, while the ranged/backline characters were hitting less frequently. Plus, it didn't really encourage tactics for us, but the illusion of tactics. It seemed cool to think the fighter and ranger were being tactical by surrounding one enemy, but really it just let the other enemies move around unimpeded and get to the cleric and wizard.
And there are so many spells and other class abilities that can give you advantage, but they rarely ever get used because people can get advantage for free. Fairie Fire goes from useful in most fights to only useful if the enemy is invisible.
If I were to put it in my games, I would probably keep it at a +1 (or +2) and probably use the movement/opportunity attack rules from 3rd. So moving around enemies became more difficult and it wasn't as easy to get flanking.