Given that the game is 8 years old, I was expecting it to look a little dated, but this looks very beautiful.
The story is a prequel to the first Halo game, so tells the tale of the Covenant's conquest of Reach. You play as a member of Noble Team; who are a team of Spartans. You may achieve many victories across the game, but there's only so much a team can do, so will ultimately be defeated; although your team members won't go down without a fight.
The gameplay is the usual Halo gameplay. What I've always liked about Halo is how often you need to switch out weapons. The ammunition is very limited, so you will be constantly depleting weapons and have to make do with what you can pick up; which means you have to adapt your strategy.
The game tries to mix up the gameplay, so there's some driving sections and slower moments where you will be walking across landscapes while your team mates talk. There's some flying missions, even one where you pilot a spacecraft in space.
As usual the game has frequent checkpoints which means you won't lose too much progress when you do die, although there were times where additional checkpoints seemed to trigger on the second attempt; i.e. they didn't trigger the first time. In terms of other bugs, I found AI controlled Spartans seemed a bit clueless when you got into vehicles. Sometimes it looked like they should take the spare vehicle but they just decided to stand there, but seemed to respawn when you reached the next section.
If you have played a previous Halo game, then I think you know exactly what to expect.