The Master of Metal!
All metal band references aside (especially Metallica references), as a six-mana Planeswalker, Tezzeret, Master of Metal is a bit underwhelming. However, in raw power of his abilities, this Tezzeret isn't bad at all! Starting with 5 loyalty isn't bad, either, even for 6 mana (4UB).
First, his +1 one essentially draws you an artifact card. That's not a bad plus ability. Plus, the other cards you reveal just go back to the bottom. The other great thing about this Tezzeret's +1 ability is that it doesn't force you to select a card from a certain number of cards from the top of your deck. You can't actually "whiff" with this ability. While card selection is typically preferable, it feels bad to miss on a card advantage ability. So this is a good start for the Master of Metal.
The -3 ability makes me very happy. It reads "Target opponent loses life equal to the number of artifacts you control." You don't need to control a bunch of artifacts for this to take a huge bite out of your opponent's life total. In EDH, I could see this ability instantly taking out players. There are plenty of artifact decks that could use this ability to fantastic effect. In Standard, Modern, et al? It's not going to be worth casting him for on its own.
The -8 "ultimate" ability is extremely powerful. Not only do you gain control of all artifacts an opponent controls, but all of their creatures, too! Plus, it's not until end of turn. It's indefinitely! This is an even better ability to use in a grindy format like EDH. It takes some work to get to, but it can be a win condition! Who doesn't like potential win conditions?
So far, the Master of Metal looks pretty good. But 6 mana is a lot to ask in Standard. In a kitchen table setting, though, this guy is a boss! If you cast Mycosynth Lattice and turn everything into artifacts, it's particularly absurd! And since the Planeswalker deck exclusives are Standard-playable, who knows... he could sneak into some list sooner or later!
So here we are with the "real" Tezzeret of Aether Revolt, Tezzeret the Schemer! He's much more aggressively costed at 4 mana instead of 6 in the case of the Master of Metal. Like the Master, the Schemer starts with 5 loyalty counters. His abilities are considerably different, however.
His +1 ability is one that I actually really like. He creates an "Etherium" token, which you can tap and sacrifice to add one mana of any color to your mana pool. They're sort of like clue tokens, except they give you mana instead of drawing you cards. I think some people may be underrating this ability. Extra mana is always good, especially in Blue!
His -2 is reminiscent of an old Liliana planeswalker ability, except instead of caring about Swamps, it cares about the number of artifacts you control. You give a creature +X/-X until end of turn where X is the number of artifacts you have in play. This ability can be used both offensively and defensively, but I see it mostly being used for removal. That's not bad for a -2 ability on a planeswalker that starts at 5 loyalty, and it only gets better as the game goes along. The Etherium token synergy is pretty good, too!
The -7 "ultimate" ability grants you a pretty cute emblem. At the beginning of each combat on your turn, a target artifact you control becomes an artifact creature with base power and toughness 5/5. It's sort of like a free Ensoul Artifact every turn, except that it's permanent without the need for an Enchant Creature card attached to it. There is one important thing I must point out, though: the emblem's effect is not optional. Once you start combat, you must choose an eligible artifact to become a creature. While this won't always be a problem, there are cases in which an opponent could pick off one of your better artifacts with creature removal in response to this ability. I still think this is a pretty good ability, regardless.
The most obvious synergy to me for this emblem is with the Kaladesh vehicles. Turning your Smuggler's Copters into permanent 5/5 flyers is extremely scary, for example. (Sadly, Copter is now banned, but there are other good vehicles, of course!) And as one player has already said, with the Etherium Tokens, you're getting free Lotus Petals every turn, which is pretty good, too! People are already talking about how well he'll work in a Blue/Black Masterwork Colossus and Aetherworks Marvel decks. There's definitely a lot to like about this guy!
From a Standard standpoint, the Schemer looks more pushed for competitive play. But the Master of Metal just has so much raw potential that even at 6 mana, if he's protected and set up correctly, he could see a lot of play. He's going to be all over EDH, at least. Both of these Tezzerets were well designed, and while I know the Master of Metal was specifically designed for the kitchen table, he's actually a lot more aggressively costed than the Chandra and Nissa exclusive planeswalkers of Kaladesh.
I give both of these planeswalkers two thumbs up, but if I have to pick a winner, I have to choose the Schemer since he can fit into a lot of competitive strategies. Personally, though, I'll be looking to cast the Master of Metal more often. Or just both!