I recently got it (and didn't even wanted it), tried at Tangled Shore activity and don't understand it. Yes, it marks one of the mobs, but usually not the one that annoys me the most and one I can easily dispose without that mark. There is a pause between marker another enemy, so it doesn't really helps clearing an area faster (tried two lost sectors, with and without mask, noticed no difference). I tried to lure attention of Fallen Vandals, yet mask ignored them, despite those snipers were chipping away pretty big chunk of my guardian's health. Secondary and tertiary perks are useless for me, at least on mask that I got (I've heard they are randomized). And don't tell me it'll help me in PvP. First, I don't plan to play it any more, as I already got Ace of Spades (gah, the horror, the horror!), and, unless Bungie lock some another nifty-sexy-memorable gun such as Ace of Spades behind PvP-wall, I won't go there. Probably won't go anyway, even if they do lock something, those six hours of fruitless attempts to score 25 headshot kills just... No. Not my cup of brew. Second, even if I go, my go-to-anywhere exotic armour is Actium War Rig (sadly its perks are strange too), and I just don't see me using anything else. Third, so it will mark my opponent in PvP, so what? Judging by my experience in PvP (which is terrible:D), my guardian either was one-shot by super, sniper, heavy, or killed pretty much fast any other way and I either saw an opponent, but couldn't kill (or even harm) him anyway, or didn't saw him at all. So, should I keep it and try to alter my play-style for PvE, or fuhgeddaboudit and enjoy my usual Dakka-Dakka?:)