Just wrapped up the Legendary campaign which took about 8.5 hours with 4 player coop. HUGELY enjoyed it. Level design and, of course, basic gameplay are an absolute joy. Weirdly, I think I actually might have preferred Prometheans to Covenant this go around; the combat loop is very different from 4, and they're brutally tough. I'll never forget when a bunch of Soldiers grabbed a Warthog and started donging on all four of us. Seriously, it was a slaughter. Must have wiped 5 times trying to kill the damn things. It never really felt unfair, just incredibly difficult, almost darksoulsy. Prometheans in 5 demand precision and strict adherence to their combat patterns; if you just try to go for headshots on everything, you're gonna have a bad, bad time. You need to strip the soldier's armor first, which is best done with an automatic weapon like the Suppressor. The Covenant received a number of interesting new tools and behaviors as well, most notably the pronounced tendency to jump fucking everywhere, stay still god dammit and let me shoot you in the head.
Ahem.
Now, the story. Much has been said of it, and while I say that it was quite good (better than most videogame stories), it does have its issues. The front half or so is rather... unfocused, which weighs things down considerably. I'm going to have to do another playthrough (probably on a lower difficulty) to really soak everything in; a lot of character details are passed on through in-mission banter, which is easy to miss when you're desperately fighting to stay alive. And then of course there's the intel, but I'd prefer not to figure that in to any general accounting of the narrative because let's be real here, most people aren't ever going to hear most of it.
But on the whole it was pretty good. Locke was more interesting than a lot of people claimed him to be, but still the weakest member of Osiris, personality wise. Buck was the strongest, of course, followed by Tanaka and Vale. Blue Team is a whole 'nother kettle of fish; they play a more... driving role in the plot (AKA it is in fact still very much the MC's story), but get less character visibility. Again, from what I saw. Will need to replay.
It does not end on anything resembling Halo 2's cliffhanger, and does not in fact end on any sort of cliffhanger at all. It's a sequel hook, to be sure, but not a cliffhanger by any reasonable definition. Let's put that to bed.