Finished the campaign, overall impressions are great, but I agree with the general consensus of it being a somewhat weak story. That and the Prometheans just aren't a fun enemy. They're annoying, they're completely free of any sort of charm(nice try with the bit on the last mission though) and most of the time I'm rolling my eyes and cussing in Spanish when I see the little rifts popping up. I sincerely hope we get to keep fighting Covenant remnants and the Prometheans don't completely replace them. I'll take any space magic I have to.
And fuck the Warden in particular. Not fun.
I liked Osiris, Locke was a little flat, I wouldn't use boring, but given his background, I think he was acted quite well. I don't hate him like some people seem to. The rest of Osiris was enjoyable. I liked Vale the most, probably because I enjoyed that her interest in the Sangheili matches my own and it's the first time in a Halo game we get to learn a tiny bit about the races of the Covenant.
Gameplay wise it's the best Halo I've played. Don't like that armor rewards are exclusive to req packs, there should have been some for achievements, completion on legendary, heroic, solo, co-op,etc. But it's not that big a deal.
Overall I'm very happy with it.