I finished the campaign on Legendary (most of them 3 player Coop, some 2 player, 3 solo) in 9h30min, which is not bad at all for a shooter. On higher difficulties I can definitely see it surpassing Halo 4's length with ease, not too sure about doubling it though.
The problem with the FIreteam AI is that you have to command them constantly in order for them to be very effective, especially on higher difficulties, which fits to 343i's "Your Team is your Weapon" motto. Independently, they still are smarter than most friendly AIs in gaming (considering they are supposed to be a substitute for real players) but there are some nuisances that occur on (almost) every level which definitely affect opinions on the AI in a negative way.
I'm not going into detail with this one but the campaign has two things (I am aware of) which Destiny was heavily criticized for. Since Halo 5's campaign is more entertaining IMO I didn't mind too much but it definitely is a fair point to criticize.
My only other complaint is the story. It's fantastic, and IMO easily the best of the series (except for one of the Destiny-like things). I was one of those who hated on the mission ratio but in the end it didn't affect the campaign (although I would like to see more Blue Team missions in future iterations). The real problem is accessibility (can't find a better word atm). Without solid lore knowledge I don't see how you can see the relevance of and understand what's going on in the story. I think this is what hurt review scores the most.
Apart from that I very much enjoyed the campaign. There's enough variety to keep you playing in one go, and a lot of replay value to make the campaign fun later on. Legendary really is difficult and challenging, although I do wish they would remove reviving from it. The levels have a lot more depth than previous Halos, in a way that it promotes exploration; huge areas, vertical play (similar to MP), hidden paths, hidden weapons, vehicles (whenever you are in a tight spot try to explore, you never know what you'll find) and a ton of collectibles (117 + 13 skulls).
The music gave me a lot of nostalgic flashes (it's loud enough this time; there are volume settings as well), the gameplay feels solid and great at 60fps, and even though you get immersed into combat because of the visuals, sounds and the huge number of enemies, sometimes you just want to stand still and look around because of how beautiful the mission looks. And no lag in Coop for me or my buddies, which we never had in previous Halos.
All in all, a huge leap forward for the franchise, but the review scores (in regard to the campaign) are justified. I wonder if 343i will release Halo 6 on Xbox One. As good as Halo 5 feels they had to cut a lot of corners to achieve the 60fps (spitscreen, dynamic resolution, certain character models being displayed at 30fps etc). On top of that, a new generation 3 years from now is quite possible IMO.