So far today, 5 cards have been spoiled. As you can imagine, some are more interesting than others:
This is a functional reprint of Armadillo Cloak. Why not just reprint the same card? Either way, this is pretty amazing, and will totally win over games in limited. It will likely see play in constructed as well, given the popularity of midrange decks and "Bant Hexproof." Cards like this just ooze with value, and I have unflinching courage that this will see play all around.
Similar to Xathrid Gorgon, This card has a large butt and deathtouch, which plays amazing well into profitable blocks in a limited format. The counter ability here can be quite relevant, depending on the synergy you have with cards like Ivy Lane Denizen, or just the Evolve and Scavenge mechanics (depending on your colors). If undealt with, this card can completely wipe your opponent's side of the board (of creatures). But, it won't see any sort of constructed play, is my guess.
Blast of Genius
There are really only two downsides to this card: the mana cost, and the fact that you must target the creature/player before drawing and discarding. Costing 6 mana actually isn't "over-costed" given the effects this card has, but unfortunately means you won't likely be playing this in the early game. And because you have to target before drawing/discarding, it's harder to know/hope to draw certain cards for maximum damage (unless you have the high-costed card in your hand already). All of this aside, this card has the potential to be pretty powerful, atleast in limited. I can't see this getting much play in constructed, but I'm not exactly a genius on the topic.
Another throwback (and *almost* functional reprint) to an old card, Morphling. Obviously, Wizards wanted to make this throwback a little different to meet the flavor of Ravnica and to conform to the current constructed environments it may see play in. Ultimately, I think they did an excellent job. The exile ability allows it to avoid removal spells, the unblockability makes this card fit into a variety of decks (like "Bant Hexproof").
Basically, it's a 3/1 haste for 2 mana. Not bad at all, definitely fits in with the Rakdos theme - and definitely a better creature than Rakdos Shred-Freak. This likely won't see much constructed play, as it's a strictly worse Strangleroot Geist, but I could be wrong.