It will be interesting to see how Companions play out. Personally I think the current ones are extremely overrated, but I can understand why everyone wants to try them out, and if the mechanic is here to stay, it's just a matter of time before a broken Companion is printed. Initially, I hated the mechanic, but now I'm not so sure actually. It makes for some diversity in deckbuilding.
Companions have a lot of potential to improve the game's problem with variance as well as the game's problem with every good card being played in every deck. Its a very well designed mechanic.
The current ones seem unlikely to completely break anything, but I think they are playable and it seems that anything Vintage playable these days can have a huge impact on the format.
Gyruda, Doom of Depths has absolutely created a T1 deck in other formats as a sorta alternate version of belcher. I think it's hard for us to imagine that happening here because you have to give up force of will but in those other formats he can T1 just off of being cast and we do have all the tools here to make it happen, as the deck is basically just fast mana, Cavern of souls, and a chain of creatures he can bring back in.
Don't underestimate them is all I'm saying. I think there are some that are clearly meh like the WG one, but starting in your opener is such a big gain it is hard to ignore.
@ajfirecracker For the deck to have a 95% success rate of milling both decks, you need 31+ creatures. To just get a board of 18+ power on turn 1 reliably, you need 22+ creatures. I've been testing it and it is solid, but absolutely dies to grafdigger cage. It is also really hurt by plow hitting Gyruda in response to the first mill, as there's nothing to clone at that point unless you hit a 2nd Gyruda in the top 4 cards. Losing misstep, FoW and FoN means you can't stop grafdigger at all. It's a powerful deck, but I think hollowvine hits 12-18 power on turn 1 fairly well already and has the bonus of 12 broken pitch spells main.
@protoaddict I still think Zirda is the most easily broken at 3 mana with a negligible requirement and 8x monoliths available in vintage. "Big artifact mana" decks are already a thing with PO, so there's the precedence of that style of deck already. Zirda basically just lets you build a deck where "channel" is always in your opener.
I scoffed at Gyruda at first, but he does have some great ability to win on T1 with 4x defense grid/4x grim monolith main. It's kinda fragile though in that all the rod effects that nerf belcher also nerf Gyruda (at least for many turns), as well as grafdigger and plow and REB being viable answers that would wiff vs belcher.