Most groups I've been in as a player did the advantage rule, and I'm mostly fine with that. However as a DM I felt like it is just too easy to set up, and decided it wasn't working for me when I had a Rogue who seemed to just never miss the sneak attack in large part because of it and, of course, crit those sneak attacks twice as often. It seemed to just overwhelm a lot of the fights once the player learned to exploit flanking to the hilt (of course looking back that particular Rogue should have had disadvantage through most of that particular dungeon due to lacking darkvision, but I didn't notice that at the time).
Currently I'm experimenting with having flanking add an extra d4 to the attack roll, potentially stacking with advantage (or disadvantage). Worth getting, and incentivizes getting into melee and taking care in positioning, but it doesn't make advantage nearly so ubiquitous, or step on the toes of various other ways to get advantage. None of the players in that game have really gotten into exploiting flanking so I can't really say much for the results other than that it has not been a problem so far.
The theory behind a d4 rather than a static modifier was mostly just that I like rolling dice and having it be unpredictable, but also because it feels like a more distinct and memorable rule and because rolling an extra die feels more in spirit like advantage, without actually bringing in all the rules significance advantage has.