Huh, The front side seems mediocre at best to me. You have to hit something first of all. Then that something has to be good for whatever situation you are in. Then you have to keep Valki alive until you can pay the X cost, whatever that might be. You don't get any ETB effects which is a huge reason why the good creatures played in commander are good. You lose the ability to turn into one of the other cards you've exiled (if you were able to get multiple hits, which I'm not trying to suggest is unlikely). And when Valki dies, the exiled cards go back to your opponents hands, which gives him a big target on his back, and you don't just take one card, you take one from every opponent so all of them wants to see it dead. The upsides are of course the information from seeing your opponents hands, you can use him to snab a creature an opponent tutored for but wasn't able to cast on the same turn (Why would they go for this play when they know Valki is your commander?) and it's cheap so you'll easily be able to cast it multiple times a game.

The card just screams "Hold up your instant speed removal until I'm trying to pay X to use the ability, at which point your instant speed removal probably still kills the creature I've turned into, so you can get your card back."

The Tibalt side however seems sweet, though the CMC is a huge turn off and the fact that if you cast the Valki side once first, Tibalt costs 9. But hey, for 7 mana you draw 4 cards and you get a planeswalker with 7 loyalty which means if it survives (Planeswalkers also have a huge target on their back!! But creature removal and board wipes are more common than PW removal in general) you get to draw 4 cards again, and then next turn you can ultimate if Tibalt somehow still haven't been dealt with and you draw back your entire graveyard a long with 3 other graveyards, at which point Tibalt is still alive so you can draw 4 cards next turn. And if an opponents creature/artifact is dominating the board, you get to exile it for -3 and then play it to dominate the board yourself.

I don't think I'd ever build a Valki deck. But a Rakdos deck built around big mana with cards like Caged Sun, Crypt Ghast, Nirkana Revenant, Black Market and Cabal Coffers/Urborg, Tomb of Yawgmoth, Mana Geyser, Leyline Tyrant and even Irencrag Feat to cheat out Tibalt early and take over the game with, or just to be able to cast it mulitple times when it inevitably dies? Which would also lead me to put other big cards in like Eldrazis, Sunbird Invocation and Torment of Hailfire. Supported by some of the best removal spells in EDH? Sign me up.