Dark Ascension brings us an interesting mythic artifact in a block that has little or nothing to do with artifacts:
There are literally zero cards in either Innistrad or Dark Ascension that allow for an efficient method for you to sacrifice the Helvault. Meaning, within Innistrad Block, Helvault's first ability is severely neutered if you can never return your creatures to play after you've "saved" them. The only way for you to get rid of it would be directly with something like Naturalize, Ancient Grudge, or Bramblecrush. What this card's first ability is something like Arcbound Ravager or Throne of Geth to be used as an artifact sacrifice outlet. That by itself is an interesting design choice. The Helvault's second ability on the other hand is crazy good.
The Helvault's second ability allowed for a reasonably costed, repeatable ability to constantly exile all of your opponent's creatures. The fact that it's legendary is the very least Wizard's could have done to keep its power level in check.
This card will be played in multiple formats.
The most interesting thing about the Helvualt's design is that even though it has two similar abilities, it seems like an either/or scenario. You're either going to try and combo the Helvault to repeat "comes onto the battlefield" and "leaves the battlefield" abilities from your own creatures multiple times or you're going to use it as a repeatable source of removal as you attempt to control the board. I haven't fully wrapped my brain around different deck ideas yet... but let's go big on this one.
How fun would a Helvault + Obliterate combination be to execute?
It's simple really: Fill your Helvault with a fatty or two, pop it with an Obliterate, send everything to hell, and unleash your aforementioned fatties back into play.
If you don't want to go casual, I can see Helvault being played almost in place of Karn Liberated in some decks. If Karn can see competitive play, so can Helvault.