I really enjoyed the first five-ten hours but was pretty disappointed after that. Once you've screwed around in the open world for a few hours, the game really doesn't throw many new ideas at you. The last like 5 missions that are all linear were really boring and too many of the boss fights were bullet sponges.
It felt like if they came up with one or two more grapple-hook level tools, then polished a few more things, didn't repeat the same three environments the whole game, it would have been something really special. As it stands, it really falls short of what I thought it was trying to be. It's hard to finish the game and not feel like they were just trying to reach a minimal viable product.
The story, while very much about saving the galaxy, is very much about overcoming grief for all three if our main characters and I think it's kinda nice. I'm interested to see where it goes and hope they don't drop the ball.
The actual missions themselves are very repetitive. The only one that sticks out is the first mission on the ship, and the one leading up to Escharum boss fight.
The real variety is in the open world. The big bases you can destroy are all different enough to justify doing them all. That being said, the lack of biome diversity is extremely noticeable and could have been bumped up a bit. The Graveyard with the AA guns and the swampy parts in the last section were nice but not enough of a change to really engage me. The swamp could have been played up way more especially.
The saving grace is that playing the game is fun. The abilities you have work seamlessly, and traversing the world with boost and grapple makes me wish it was in every Halo. The weapon sandbox is fun to fight against but, unlike multiplayer, I feel like you're genuinely always better off with just a BR and Commando.