I've played coop twice and i didn't really like it. It's too easy and it seems that when you quit you have to start from the beginning always.
multiplayer is pretty nice. Its doesn't seem to have OP weapons and the game modes are quiet interesting. The Create a class looks sort of dodgy, but once you get used to it you can skim through it pretty quickly. The streaks are gained with points, just like BO2, but they are awarded in a different way:
You are forced to use 3 killstreaks, each of them in a different tier, and you can choose them. Once you gain enough points for the 1st killstreak nad you use it, your streak empties out (if the streak is worth 250p and you use it when you have 350p you will end up with 100p). If you want to use the last pointstreak (the most deadly one) you need to get enough points for it. Once you use it you will end up with 0 points left. When you die you get a small portion of your poinstreak taken away, but you still get some points remaining (lets say it takes only 50p away per death).
And the last thing is the decoders. Whilst you level up you will earn these decoders (iirc they where named like that). Decoders have to be decoded and they award weapons, equipment, perks or EXP its random depending on the decoder's length. The bad side to this is that if you get a 10 minute decoder, you need to leave your decoder menu on the multiplayer menu open for 10 minutes until its decoded. 10 minutes is the minimum, 16 hours is the maximum iirc. So you can end up leaving you menu open for 16 hours just for that item.