I think I'm about 3/4ths of the way through the game, 50% of my time has been co-op (34 hours co-op, 36 hours solo). It's still fun.
It absolutely _IS_ repetitive, insofar as you're killing bad people. It gives you a number of ways to do that though. I imagine you'd need to get some personal joy out of clearing a base solo, or entering and exiting undetected.. because the game doesn't reward or cheer for you when you do so. To me, I love trying to do ridiculous stuff, and then when it fails, survive the firefight.
Co-op lets you do so many random things, it's a lot of fun, but a lot of that will depend on the quality of your friends.
If you approach the game as a standard "do the missions, advance the story, finish the game" I could see it becoming really tedious.
None (well, most) of the randomly scattered things around the map are required to collect, but you'll end up getting most of them I imagine. I like clearing a province 100% so I end up getting them all.. but that's me, I wouldn't recommend it unless you really like doing it.
The AI isn't terrible, they're not great either. I use them as a backup if things go sideways. They're meant to emulate real people in co-op, so they get some godly powers because there is no 1:1 replication of real life friends.
I started a new character (A girl!) last night and played through the first few missions.. still amazing.