I'm really loving mutate if I'm honest... I feel like it's just another cool aspect of the game that should be explored and experimented with, and it doesn't have to be game-changing, although it can be. The Apex creatures are really sweet, especially Nethroi and Illuna, my two favourites. I think that any Voltron deck could have a bit of Mutate, as everything is already invested into that one creature.
The thing I don't like about mutate is how it costs you numbers of creatures for boosts. But, then again, that is unavoidable. My favourite part of mutate, however, is the "whenever this creature mutates" trigger. Epecially in cases like
Parcelbeast,
Everquill Phoenix,
Migratory Greathorn, and the Apexes, the results can make or break a game, and I think it's really cool that the whole stack of creatures is triggered. It just seems satisfying.
A lot of people were saying that there isn't much hexproof in standard, but that's not the problem for me. The problem is that hexproof (except for
Paradise Druid) is too expensive. You have cards like
Humongulus and
Wingfold Pteron as well as
Dream Trawler to base creature stacks on, but they're all late-game.
So anyway, I'd say a 7/10 for mutate, because though it is really fun and you can make some amazing combinations with ridiculous triggers, it costs you a lot of creatures to make a large stack and can be very risky, as you have a lot of mana and cards in one spot.