If there was ever a game that suffered from bipolar disorder, it's Halo 5: Guardians. The multiplayer serves as definitive proof that 343i were willing to listen to the criticism from fans surrounding Halo 4, but the campaign (specifically the story) is a complete disaster.
Multiplayer (tl;dr it's good)
Halo 5's multiplayer comes in two distinct forms: "Arena" and "Warzone". Both are fantastic.
Arena is a real return-to-form for the competitive Halo multiplayer experience. Gone are the loadouts from Halo Reach and Halo 4 in favour of the equal starts that everyone wanted. Sprint is still there but reworked from Halo 4 so that getting shot stops your sprint and if you're sprinting your shields won't recharge. Personally I'd prefer it not be there and base movement was increased by 5-10% across the board, but I'm totally okay with how it is now. The new Ground Pound, Spartan Charge and Thrusters are all good additions. They make Halo 5 feel like a genuine sequel rather than just a rehash. It uses a CSR ranking system similar to League of Legends. After playing 10 qualifying matches in a playlist, you get placed a division (Bronze, Silver, Gold, Platinum, Diamond, Onyx) at tier 1-6 depending on how many of those matches you won while taking into account the CSR of players you were versing. After that you play matches with and against players of similar ranking to you. Winning matches moves you up, losing can drop you. I can safely say that it's the best competitive Halo has ever been. EVER. Unfortunately, as of writing this, there is a serious problem with a lack of gametypes available. Currently we have: Slayer, CTF, Strongholds (Domination) and Breakout (one life per round, no shield, neutral flag). Yes you read that correctly, there is no Infection. Or Oddball. Or Assault. 343i have stated multiple times that they're working on adding gametypes in the future as part of the monthly free updates (which also include new maps, weapons, cosmetics, etc) but so far these updates have been pretty disappointing as they seem to pander to the MLG crowd more than the core Halo playerbase.
Warzone is a much more 'casual' experience, but in the best way possible. It's Big Team Battle on steroids. If you've played Titanfall then it should feel familiar, but better in every way. 12v12, each team has a Home Base and there's three bases that players fight to control. After clearing out the AI Covenant/Prometheans from a base and capturing it, AI Marines for your team with spawn in to help defend it from the enemy. There's also a variety of "Boss" AI enemies that spawn in throughout the match, offering a large 'VP' reward to the team that manages to get the last shot on them. In order to win the match, a team must either accumulate 1,000 VP or control all three neutral bases, exposing the opposing team's Home Base Core and destroy it. The twist here is instead of power weapons/vehicles spawning around the map for players to use, they must 'earn' them. Each player has an individual level, increased by killing enemy Spartans, AI, or capturing bases, which allows them access to call in "REQs" (A Shotgun can be spawned at level 3, a Sniper Rifle at 6, a Phaeton Gunship at 8, etc.) But "energy", which is generated over time, acts as the currency used to 'buy' these REQs. The amount of energy each REQ costs is equal to the level required to have access to them, and your maximum energy is capped at whatever level you are at any given time. The system is spectacular and it really makes the "Warzone" feel like it's constantly escalating. Everyone starts with an Assault Rifle and a Magnum but towards the end of the match there's tanks, rockets and all other kinds of mayhem happening.
One common criticism I hear about the REQ system is how instead of every player having access to every weapon/vehicle all the time, you can only call in REQs from your pool of them. Basically how this system works is on top of earning XP from completed matches you also earn RP, which is used to purchase REQ packs from an in game store. What you get from these packs is completely randomised and if you run out of Sniper Rifle REQ cards, you have to buy more packs and hope you get some more. You can also use real money to buy REQ packs. The complaint I often hear is how it's essentially microtransactions, but this complaint is totally based on ignorance because the rate RP is earned is very generous (one Warzone match will earn you more than enough RP to buy a Bronze pack).
Campaign (tl;dr it's shit)
God fucking damn I fell for it. Halo 4 warned me that the Halo saga would be shit from here on out but I fell for the marketing. The Hunt the Truth audio series? The identical but opposite pair of trailers that aired during The Walking Dead's 5th season finale? The box art? All lies. This game isn't about the Master Chief being framed for crimes against humanity. It's a prologue to Halo 6. Granted, the level design is still top-tier, the new co-operative squad mechanics are awesome and Legendary difficulty is a decent challenge but fuck me the story is complete garbage. Of the 15 missions, you only play 3 (THREE!) as the Master Chief and Blue Team. At least these missions are the longest ones but Halo 2 had a much healthier balance of Chief/Arbiter missions. The rest you play as Spartan Locke, following Master Chief through his journey to find Cortana. Locke is a terrible character, somehow he has even less personality than Chief. The rest of 'Fireteam Osiris' (except Buck from Halo 3: ODST who honestly feels like he doesn't want to be there and would rather side with the Chief) are all just as one dimensional as Locke is. Vale's only memorable contribution is how impressed she is that The Arbiter has female Sangheili fighting with the males (although you never actually see female Elites fighting, there's only one who provides intel to you via comms) and Tanaka is as generic a black female character can be.
Bonnie Ross, the head of 343i, said she wants to run the series like she's George Lucas (I'm not kidding, she actually fucking said that) and by taking a beloved series and shitting on it she's doing a pretty fucking good job thus far.
The "Big Showdown" between Chief and Locke happens halfway through the game and serves absolutely no purpose other than leaving Chief with a crack on his visor. The fistfight also looks like two people practicing hand to hand combat rather than actually trying to beat each other up. It's cringeworthy as fuck. The game ends on a Halo 2-tier cliffhanger. Holy shit I'm so mad I can't even continue writing about this shit without having a stroke.
At least the multiplayer is really good.
Halo 2 GOTY in both 2004 AND 2014