This might be borderline playable in Standard. But it's playable in Legacy Eldrazi Stompy decks and has seen play in Eldrazi Tron decks in Modern. However, Endbringer's best home is likely in Commander.
Producing true colorless mana is rarely an issue in Commander and the abilities are powerful enough that they can fit into a Commander deck fairly easily.Many Commander players were excited about this card when it was first spoiled. But many competitive EDH players were not.
This card seems like a staple for Commander, though, doesn't it? Colorless card draw, in particular, is especially useful. Tap your Sol Ring or other colorless mana rock and tap this guy, free card! Best of all, while colorless mana is required to cast him and use two out of three of the abilities, it's such a small cost and pretty much any Commander deck can use him. In a four-player game, you're often going to be drawing 1-3 extra cards before it's your turn again. The colorless requirement does balance it out a bit, but this guy is pure card advantage. At the very least, he can ping, which is not nothing.
The great thing about Endbringer is that his abilities can fit into another deck. It's all about just finding a slot for him. Early on, Animar, Soul of Elements (who can cast him for a single Colorless mana on many occasions) and Kruphix, God of Horizons are the two Commanders were mentioned as solid homes for this rare Eldrazi. Really, though, he can go just about anywhere.
While some Animar and Kruphix lists indeed play him, he hasn't become widely adopted by those Commanders. For the most part, he has only found consistent homes in Eldrazi Tribal decks, such as Kozilek, the Great Distortion, Ulamog, the Ceaseless Hunger, Emrakul, the Promised One, and Kozilek, Butcher of Truth.
Could he ever become a new staple in the format? Even as a 6 mana creature, that seems possible. Like many of his colorless Eldrazi buddies from Oath of the Gatewatch, this guy should make himself known in the format for quite some time.