So now that I've finished it, a few thoughts on the SP. This one is definitely a departure from the standard CoD forumla. 90% of the missions you play as the main protagonist, with a few departures. You have a face, voice, and your motivations are not clear. You're seriously fucked up, you start the game strapped to a chair. That isn't a spoiler, it's literally the menu screen.
Now while they've tried very hard to craft and tell a proper story, it's going to be very confusing to some, and a lot of this comes from the confusion of the protagonist. If you're basically learning the story through the first person, and the protagonist is confused, you will necessarily be as well. It definitely came together towards the end for me, but I'm not sure how much of that is due to the fact that I already knew what a certain aspect of the story was. They make a big deal about it like it's some sort of mystery, but it's actually a pretty well known spy thing, and I think that unless you're aware of what it is, it's going to seem really hokey and scifi-ish, and probably seem utterly ridiculous. Without knowing what that's all about ahead of time, it's like a layer of confusion upon confusion. The way I see it, the protagonist surely knows what it is, so you're doing yourself no great disservice by doing some quick googling.
Overall they maintain the level of excitement you expect from a CoD campaign, and the missions get more and more interesting as you progress. Definitely worth your time to play through.