Menacing: This ability can only be placed on a melee weapon. This weapon property helps allies deal with flanked foes. When the wielder is to a creature that is being flanked by an ally, the flanking bonus on attack rolls for all flanking allies increases by +2. This ability works even if the wielder is not one of the characters flanking the creature.
Outflank:
Benefit: Whenever you and an ally who also has this feat are flanking the same creature, your flanking bonus on attack rolls increases to +4. In addition, whenever you score a critical hit against the flanked creature, it provokes an attack of opportunity from your ally.
So:
Menacing requires you to be adjacent -- which means it is useless on reach weapons -- it doesn't give you the bonus if you aren't flanking, and it increases whatever bonus people get for flanking by +2.
Outflank changes the base bonus to +4 instead of +2 which would stack with menacing. Also when you critical with outflank you give your buddy an attack as well. With Gang Up and Outflank you and two of your friends (provided you all had outflank) could flank the same creature no matter where you are standing.
If you had a menacning scimitar and you are flanking then you would have a +6 bonus to hit (as would your ally) and on a critical hit would give your buddy an AoO of his own (and if he criticals with that you get an AoO of your own too).
So why outflank instead of menacing?
Menacing is cheap -- on a +1 weapon. But when you have a +5 weapon and you also want keen, speed and (something else) on it then it's much more expensive. Also you must be adjacent for menacing to work.
Outflank works as long as you flank. Also it gives the extra attack on a critical hit. Finally you might be of a class like cavalier where you need a teamwork feat to take (and can grant it to others). Also an Inquisitor makes great use of Outflank as well since he doesn't rely on others for his bonuses.
sounds like a keen, menacing Scimitar is the best.weaponfor an Outlflank granting cavalier then....
Still seems to be a great Enchancement for buffers. Gangup bards for example
It isn't bad -- but I would rather have another +1 to hit in most instances, and the requirement of being adjacent really kills this enhancement.
Consider: I step beside the monster with my menacing weapon -- now he five foot steps away from me. Great on my turn... not so much otherwise -- also it doesn't work with lunge at all (which stinks).
Now is it useful? Oh yeah. But it's not perfect (which it shouldn't be for a little +1 enhancement of course).