Imho Death Flayer is musthave pet,
but for serious PvP and premades only, when you're using 20\31\0 build with bestial swiftness, especially if you playing Horde. Why? Because of viper sting protection. It's the pain in the ass to dispel wortthless stacks of scorpid poison instead of viper sting, dispeller always must to dispel himself (otherwise viper sting will drain him to oom) which leads to waste mana and global cooldowns on dispel.
Advantages:-That's the only way to kill paladin or restoration shaman. Scorpids are the real pain for healer classes.
-Death Flayer is better than BT for the situations where hunter must spam Viper Stings on healers, like fights on BS, WSG mid fight or something like this. Bestial Swiftness is musthave talent with this pet so he can always get uptime on healers.
-Death Flayer is very easy to tame.
-Death Flayer is simply better than any other scorpid because of 1.6 attack speed.
-5 stacks of scorpid poison is a good damage bonus when you kiting somebody on 41+ yards, esp vs plates.
-Black scorpid pet makes you a TBC gladiator fella
Disadvantages:-Scorpids don't have Dash or Dive, so without Bestial Swiftness that pet is useless.
-Bad pushback attack speed (1.6). If your main goal is to interrupt spellcasting, choose BT.
-There's a bug: Scorpids do not reapply 5'th stack of scorpid poison. So if the target already have 5 stacks, you'll have to push a button or macro yourself to re-apply it.
-He's lvl 11. Need to tame various pets to get all abilities Claw 8 from Winterspring, and Scorpid Poison 4 from BS\Silithus and then lvl him up to 60.
Please note that scorpid was the ONLY pvp pet choice back in tbc arenas.