Finally finished the campaign. GOTY for me, atm. That said, It wasn't all that much longer than previous campaigns - the game tells me I did it in 9 hours, but I will definitely go through it again on a harder difficulty and try to be more thorough (only found around 70% of collectables). Semi-open world thingie wasn't essential, but it definitely gave the game some freshness. The most important thing is that linear levels felt better than ever, because they somehow managed to improve that "Hey, it is a bunch of covers in this room. The fight starts in 3...2...1." feeling that got kind of old by the fourth game. Also, the story was great and it is nice to see some thematic depth added to the game - basically the whole game felt like a new generation has to fix the fuck-ups of the previous one. Sounds familiar?
I played on X and I still think the graphics were amazing, mostly because HDR is the best I have so far. I have seen people making fun of its graphics, saying it looks like a 360 game. The funny thing is that it kind of does, because the developers clearly decided to keep the visual continuity from the previous games, but did a lot of subtle stuff around it lightyears ahead of what 360 could do. It basically looks like the best ever remaster of a 360 game. I understand if it is a turn off for many. Personally, I loved it.
P.S. Played the whole thing in online mode. Had the checkpoint bug only once. Other than that, didn't encounter any bugs.