Doug Beyer heralds the start of official Shards of Alara previews with this article that sets up the flavor of the world, from the point of view of planeswalker Sarkhan Vol, whose art was previewed back when Wizards introduced Mythic Rares. Now we have the full stats:

First off, nice green-red flavor there (especially given Sarkhan's backstory -- go read the article for more on that).
Will this fit into tournament decks? Well, Sarkhan's positive ability is potentially quite solid if you build around it. You can imagine Sarkhan enabling a rapid assault by a bunch of Goblins spawned from a Siege-Gang Commander, or just being slotted into an Alara-era remake of the tokens deck. His second ability is also potentially quite useful, especially as it can make Sarkhan a game-turning or game-ending topdeck. "Play Sarkhan. Steal your dude, kill you with it." The last ability is, at a glance, pretty frightening, as it's more or less a "win next turn" power. I'd be more worried about its impact but for the fact that so far, Planeswalkers haven't really been winning most of the time through their ultimate powers (consider what happens when Chandra goes off, for example). That said, I did round out a few games at the last PTQ I attended by using Liliana's ultimate, so it can happen, and it pretty much does ice a game in her case.
Once again, rather blind of whatever else is showing up in Shards, Sarkhan seems like a pretty reasonable Planeswalker who will find his way into some R/G big-mana style decks that rely on token generators, and he may also serve in pure tokens decks as well.
Now, although I've previewed the blue planeswalker from Shards, I haven't really touched on the already-previewed Ajani from this set:

I can't comment much on Ajani's flavor here, except that he seems a little pissed off, and the "Vengeant" label suggests that things haven't gone well for him lately. I imagine they'll flesh that out in time.
On the gameplay side, your eyes were probably drawn immediately to his second ability, which amounts to "Pay two loyalty: Lightning Helix." It's a step up from Chandra's power in that you gain life and can point 3 damage at your opponent, but it's a step down because Chandra can come into play and really save you by burning down a big creature. Ajani, for example, can't kill a Colossus.
But...he can keep it tapped down. Ajani's first ability is interesting enough, and gives him some potential to keep the opponent stalled out long enough for you to stabilize. It also builds up, albeit quite slowly, to his ultimate ability. I'm completely unsure of how good a slow-moving, one-sided Armageddon will be. Although it's not an "I win" on its own, it's pretty solid if you have the right things in play at the time (maybe Chandra, eh?). With a four-turn clock from the turn you play Ajani, however, there's a ton of time for your opponent to do something about the situation. So it's nasty, but not nearly as immediate as a Garruk overrun or even a Sarkhan dragon blitz.
Doug Beyer tells us that there's one more planeswalker left in this set. So far, we have blue, red-white, and red-green, so if we can guess at anything, it might be that the last one will be black or some combination of black and other. We'll see.
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