Good, but is held back by various problems.
In terms of strengths, the game is very good looking. The environments are beautiful and very well done. To no surprise the music is great and compliments the environments, as halo is known for doing. The game has its high intensity moments, both in what you're doing and emotionally. But, past all that there are problems that persist through the game. First off, the level design is kind of cheap. Without a weapon that has long range, it will be obvious early on that enemies will be picking at you from far away, or the effective range of shorter range weapons will not kill quick enough to prevent a death. Secondly, the main enemy in this game doesn't make a huge appearance throughout. Speaking about that, the prometheans aren't varied in their enemy type. There are three types of prometheans, the only difference being that two of them switch weapons here and there, but this only leads to them having cheaper weapons. The only truly emotionally gripping part is the end(legendary difficulty adds virtually nothing). There's also a part at the end that will leave the player wondering "How is that-when did he?." It is in terms of enemies, level design, and story, the second weakest campaign. The ending to a story, which halfway through is obvious, can't carry the whole story.
Spartan-ops is a very refreshing spiritual replacement to firefight. You spawn with whatever classes you have created in your spartan career. You play as crimson team, a team of spartans that are the best of the best among their teams. You play on various parts of maps completing objectives that come down to, activate, deactivate, defend, neutralize, clear out the area, and in one awesome case, survive. You fight through waves of enemies completing these various objectives, while getting assistance in ordinance drops. It's basically a horde mode, fused with parts of the campaign. Very good fun, better with friends as most things are.
The multiplayer is like halo 3 mixed with reach. It is a new experience that brings some good, some bad, with it. The good is that this experience does feel refreshing, and does have a lot of fun new weapons to mess around with. But with these new weapons has come balancing issues. Along with that, the maps for non-big team modes are very small and will result in bad spawns. The maps on big team are so large and separated, that they result in trench warfare with the two sides staying on their sides taking potshots. The ordinance drops, while fun, lead to obvious balance issues and deaths that will feel less than fair. Regicide is a fun new mode, that partially makes up for the lack of playlist options.
As a last little part of the game, forge is fun but is far more limited than the previous ones. It has more options than three, but much less than reach.