For me it really just depends on the players. I don't mind having a rule that everybody (including monsters) can use. Kobolds, wolves, rogues, and other creatures already essentially have a flanking rule. That's why I don't mind considering it for the right circumstances. Yes, there are characters that essentially can't be flanked, but I would consider that the result of an ability or feat.
So for my campaign I'd be OK with it being the result of a feat. But I also like to see the players think about their tactics (and my monsters use tactics and terrain to their advantage too). Flanking is an easy concept, and pretty easy to include. If you think advantage is too much, then do a +1 or +2 bonus.
Part of my approach is because I have two players that learned with the 4th Edition. We sometimes use minis as a visual aid, but I don't want them to be counting squares and moving their minis to specific positions, etc. So part of the reason for including it in my current campaign is to allow them to think in a similar way, but to get away from the war game approach with the minis.
The way I worded my option is to take into account that when you gang up on an opponent, it's more difficult for them to defend as well against multiple opponents. But that doesn't apply if you are also adjacent to several opponents yourself, since you have the same problem and can't be as focused on the advantage you gain.
The reason I keep falling back on advantage/disadvantage is because it's a core mechanic in the rules. Aside from that, the whole point about tactics in combat is to get the upper hand - that is, advantage.