Planeswalkers have historically been a very powerful class of cards. Looking over their history, virtually all of them have found their way into competitive decks at one time or another. Sure, there have been duds here and there, like Tibalt and nearly every Chandra before the latest incarnation, but if you're looking for the powerful cards in a new set, the planeswalkers are generally a good place to look.
The strength of planeswalkers lies in their ability to continue to generate an advantage every turn they're in play. Powerful planeswalkers are those that can be played on an empty board and provide an immediate impact and threaten to take over a game if left unchecked. The best are those that offer not only steady value but also flexibility depending on the game state and have abilities that can be used to defend themselves. My all-time favorite planeswalkers are Elspeth, Knight-Errant; Domri Rade; and Garruk Relentless because they can break a game open with either of their abilities—sometimes you're in the market for creatures, and sometimes you're looking to fly or fight.
Well, here's the next in that noble line:
At first glance, Xenagos, the Reveler looks a lot like Garruk Relentless since he offers the ability to make a 2/2 creature every turn. He is much more aggressive than Garruk, though, since these Satyrs have haste. One of the big reasons I was a huge fan of Elspeth when U/W Control decks with Jace were all the rage is just how powerful she was at fighting against other planeswalkers. A planeswalker with the ability to immediately threaten opposing planeswalkers is a big deal. Your opponent will frequently feel safe playing their planeswalker on what looks to be a stable board, and you'll able to punish them by taking it out (or taking it low at least) while substantially advancing your board in the process.
Xenagos, the Reveler isn't just good at beating down though—he's also a mana generation tool. I can imagine a deck using this guy and Garruk, Caller of Beasts alongside a bunch of mana guys to dig for and play huge creatures incredibly quickly. The fact that both the +1 ability and the ultimate can help spit out giant monsters may make this card seem like it would be best used in a ramp-style deck, but the ability to produce 2/2 haste creatures gives it another dimension that makes it feel right at home in a midrange deck with some big finishers as well.
My first thought was that his +1 ability seemed rather out of place next to his 0 ability since most decks that want a 2/2 haste creature aren't in the market for a ton of extra mana, but the new monstrosity and bestow mechanics in Theros provide excellent mana sinks even if you aren't playing with a bunch of really expensive creatures. I can just imagine a board stall in which one player has this guy along with Polukranos, World Eater and a ton of creatures. One activation can mean absolutely wiping out the entire opposing team—which will probably mean the board isn't stalled anymore. Now that is monstrous!
I can tell you this—as soon as I opened my email with the spoiler for this guy, I started thinking about building a deck with him and Domri Rade. Each of them rewards you for playing with a lot of creatures, and both are at home in a deck with a bunch of mana creatures and monsters. Xenagos, the Reveler is my kind of card, and I'm sure he and I will be good friends for as long as he's in Standard.