As a series about the Halo characters and world it is dreadful, a real wasted opportunity. As a generic sci-fi adventure it is watchable but not especially good.
If you are a fan of pre 343 Industries Halo you will find it frustrating, both in characterisation and storytelling. Typical modern Hollywood, it is to the original what Rings of Power and Wheel of Time are to their sources, second rate agenda driven dross.
If you are looking for a Halo fix, "Forward unto Dawn" which is free to watch on Youtube is far better.
If you are a fan of Halo, as noted above, it is absolutely awful. Cortana lobotomizes the Chief, who spends 95% of his time out of armor (which nullifies one of the key points of Halo, you are never supposed to see the Chief's face so that you can imagine him as anyone). Toss in a completely pointless love story and the Covenant actively working with a human to activate forerunner tech....blech.
Outside of that, the CGI is mid level. Nothing great, but nothing too horrible.
I was ready to quit well before the final episode of season 1, but my wife wanted to get to the end. I couldn't finish the final episode, and walked out and let my wife finish it.
As a fan, I couldn't stand it. My wife, who is not a fan, gave it a C-/D+.