The basic problem with Plunge into DArkness is that it becomes difficult to use more than one time - especially if you are planning to get the card - the same reason why it is often difficult to use Spoils of the Vault in combo - with Draw 7s available (and the cheaper Consultation), you should be able to see through a large portion of the deck to begin with.
And in combo, the difference between one and two mana also plays in a role. The cc of two is dangerously close to meaning that it will cost a turn's worth of mana investment, which becomes dangerous when trying to avoid Duress or Mana Drain magic up.
I think any deck that would want Spoils can use this.
Problem is, few decks currently want spoils. Spoils Mask was the main one, but has obviously fallen below the ranks of viability should one want to take a tournament. The investment of life for a one card net is not optimal - particularly when trying to buy time versus Tog and the like.
you can pay 5 or so to get yourself further into your deck and snag a nice card.
Of course, this is just a slightly better impulse. Most combo decks are quite tight in their builds, and it becomes difficult to fit Plunge in simply because it is not as consistent as Consultation or even spoils. If one is to chose a larger number (say 10), it makes it difficult to use twice, and there's no gurantee of getting it.
This card will be difficult to break simply because it serves the role as utility, and is not more effecient that current versions of the Consultation or using Draw 7s to begin with.
edit: Wrong information about the card led to an inaccurate assessment that does not change the overall assessment of the card itself. My mistake.