Yes it is.
Critical damage (which you can trigger it base on the odds percentage at the equipment UI) normally do 150% of your normal damage for the highest critical damage ratio
However,
Massive Critical gives you an insane amount of critical damage
within its critical percentage odds beyond the normal 150% damage boost, with an another chance of doing an instant kill to any Normal Enemy with 2 level below you, so the "prize" that you get is the Critical Damage bonus, and the Insta-Kill as a "Bonus"
another example is my Lv. 30 Rogue using Shortsword of Frost dealt 21-24 damage (+2 Cold) with 6% critical odds, here's my critical damage output list
without Massive Crit. Skill -> 6% odds, 25-36 Damage
with Massive Crit. Skill 1 -> 6% odds, 25-48 Damage
with Massive Crit. SKill 2 -> 6% odds, 25-60 Damage
with Massive Crit. Skill 3 -> 6% odds, 25-72 Damage
you can see that the highest critical damage ratio is the only number that significantly increased when upgrading the Massive Crit. skill,
it doesn't increase the critical percentage odds at all because it's the base odds that the Massive Crit. skill effect take place
My suggestion is this: Playing the odds of he critical percentage odds, you can do that by
increasing the odds (adding Precision Shots skill and/or Precision Strikes Skill) or
increase your DPS instead (adding Rapid Fire Skill and/or Flurry Skill)
hope this explanation helps you
TL;DR -> skill only for Boss Battle / Creep cleanup