The following details reflect the Normal version (403 ilevel):

Your special weapons swings have a 2% chance to spawn a stationary tentacle on the ground.
The tentacle attacks with a mind flay, channeling for 3 seconds. The mind flay ticks every 1.5 seconds for a static value of 11155 damage.
Attacks on average 8 ticks per tentacle, causing 89,240 damage or 7,214 dps.

Pros:
Looks cool, goes great with my Vanquished Tentacle of Yog-Saron.
No Internal Cooldown (ICD).
The Mind Flay spell slows. Could have some practical use in BGs.
Gives you extra burst, whether you need it or not.
The DPS gained from a tentacle is worth the secondary stats you lose.

I've been doing a -lot- of testing with Gurthalak. Here's what I found.
It does not scale with attack power or spellpower.
It does not directly get more ticks with haste, even with bloodlust (though more haste means more tentacle procs)
It seems to inherit the player's spell hit and the Mind flay casts (not ticks) go off the spell hit table; 88+ dummy had a few misses with 8.02% melee hit and 9.4% spell hit and it missed occasionally. I also went to the 85 dummy with about 5.04% melee hit (5.9% spell hit) and never saw a miss over a sample size of 542 ticks.
It does not seem to inherit the player's melee crit as a post above me indicates. While on the 85 dummy with a lot of agility gear and no crit which pushed me to 11.62% melee crit and 1% spell crit and the same sample size of 542, 9 of those 542 ticks were crits. Assuming theres no crit suppression on the 85 dummy, this is a crit rate of about 1.66%, which is strongly in favor of inheriting the player's spell crit.
Baseline damage of normal Gurthalak proc is 11155. Each proc, assuming each cast hits casts 3 mind flays for 3 ticks each, totalling 9 ticks per proc. It inherit's damage aura's from the player that increase all damage, so a player in berserker stance will increase the damage from 11155 to 12270.5. Also, changes to the player's damage modifier won't affect mind flays already being cast (i.e. a mind flay starts casting for 1 ticks of 11155 in defensive stance, warrior goes berserker stance. it continues to tick for 11155, but the next spell it casts will benefit from berserker stance, hitting for 12270). Furthermore, it is also affected by damage-taken modifiers on the boss, be it all damage, or spell damage debuff.
LFR baseline damade is 9881. If it scales linearly to heroic, heroic -should- hit for 12429/tick.
Crits are 150%. They also inherit the player's meta gem crit bonus, increasing crit damage to 154.5%
Can confirm there is no ICD as I had up to THREE tentacles attacking the same target at the same time, and all 3 dealed their own damage (combat log easily showed this as 3 ticks of mind flay in the same second as well as numbers popping up on my screen).
If you happen to be an unholy DK, the proc's damage increases with the amount of mastery you have.
The proc does not inherit the death knight's 200% spell crit modifier; it still crits for 150% modified by crit meta.
Also weapon-based attacks not considered to come from Gurthalak will -not- proc it. This is from a sample size of 633 slams from the main hand and the only source of rage was battle shout and berserker rage, absolutely no autoattacks at all. I had gurthalak in my offhand and a garbage green in the mainhand. There were -zero- procs. HOWEVER, if you are attacking with a non-weapon-based melee attack, such as Bloodthirst, then it -CAN- proc, regardless of what hand the weapon is in as long as it is equipped.