Bogardan Hellkite has a similar ability, except that ability hit players instead of planeswalkers, as planeswalker cards didn't exist in the game when the Hellkite was first released in Time Spiral. The Hellkite cost 6RR to cast, but also had the Flash ability, meaning he could hit the board at instant speed.
So which is better: Dragonlord Atarka or Bogardan Hellkite? It's fair to say they're pretty even when you consider the cheaper mana cost and the extra power. Hellkite was only a 5/5 and didn't have trample. Considering the ramp that Green/Red Devotion decks have, the Constructed deck in which Dragonlord Atarka would fit best in, this is definitely playable. Here's a threat that not much will be able to deal with. Nothing else has 8 power or 8 toughness that can match it in the current Standard environment, at least nothing that would regularly see play.
As a commander, Dragonlord Atarka is pricey to cast, but is a huge bomb when it hits the board. To be fair, however, Atarka's previous form, Atarka, World Render, has an ability better for a commander: giving Dragons double strike when they attack. This Dragonlord would seem to be better as a complement to a deck piloted by World Render. Of course, there are plenty of decks in Commander happy to include a big beater like this, especially most Dragon decks, and perhaps Mayael the Anima would invite Atarka to join her 5-plus power parade. All in all, Dragonlord Atarka is very playable, even at that high of a casting cost, simply due to the power level. This Dragonlord should be Constructed playable and a casual favorite, which is good for any mythic rare Dragon.