From Jeremiah Rudolph's primer on mtgthesource:
Turbo Eldrazi, or 12post as most folks will name it, is a ramp deck with a very narrow core card list. Because of this the potential cards are large and because of the inordinate amount of mana you can generate the cardpool is increasingly magnified.Because this core constitutes a good chunk of your manabase it leaves the remainder of the deck up for considerable variation. The weakness of a deck where you are strongly encouraged to plant one of 4 cards into play, Cloudpost, is when your mana and selection are pressured. Thus increasing land tutors and hand improvement is paramount. That leaves us with an addition of some fixing. It is in this last portion that the most variation is introduced and often the modifications to this will adapt the deck to your particular meta.
Since combo will give you the hardest time and the core of your deck is your mana base, you can afford to side in heavy amounts of cards without diluting your deck's potency, merely its kill's consistency. Since against combo you rarely care about winning before them, this is not a problem.
The concept of the deck is simple enough. You sacrifice early tempo and board control for late game overwhelming advantage through uncounterable win conditions, removal, and damage.
The difficulty of the deck is that your average archetype in legacy involves 2-3 decisions per turn with diminishing choices after the first three turns. Turbo Eldrazi is the exact opposite. You often have 1-2 decisions in the first two turns, and every turn thereafter the number of choices increases exponentially. By your turn 4-5 you often have to make more decisions than a normal deck makes in 3 games of magic. Compounded on this increase in number of choices, your first few choices have incredibly large impact for the remainder of the game. As a teammate Scott Hughes aptly stated, "With this deck you can play the wrong land on turn 1 and lose the game for yourself."
That being said, the most successful variant of the deck is as a Control deck that has the ability to combo finish.
Combo, especially random combo, is Eldrazi's hardest matchup. The reason is that restricting both the timeframe to build up mana and the avenue to interrupt their combo is the best way to counteract the holistic and inevitable defeat that Turbo Eldrazi brings to a matchup. Waiting will always result in a loss against Turbo Eldrazi, there is no competitive deck with a greater endgame.
