I've played many hours of Far Cry 5 co-op, both with my partner and with colleagues. A lot of the time it's been a blast, except when you're trying to actually get things done.
There's a pretty major caveat to the co-operative experience, as there was with Far Cry 4: only the host player's progress is saved. This is pretty harsh on the joining player. There are a hell of a lot of missions to mop up, so playing in co-op means twice as much busywork and having to do story missions (and their cutscenes) twice over.
It's understandable, as the resistance-points system means that the story progress is tied in with side-missions and other open-world progress. One imagines that trying to keep track of that for both players would be extremely technically difficult. But it does make it less likely that you'll want to spend a lot of time playing with a someone else, because every hour you spend in their game is an hour you're not making progress in yours.
Almost any game is more fun with a friend, but when that game involves a lot of open-world systems that can get a little confused with even one player, two players can make it go haywire. I found that story missions that were obviously balanced for the solo player tended to go horribly wrong with two people involved; more than once we were left shuffling our feet because a cut-scene wouldn't trigger.
If one or the other of us died, sometimes missions would get stuck. A lot of this will be ironed out in patches, I assume, but right now it makes getting things done in co-op very irritating.
The easy solution here is to ignore the missions — all of them — while playing co-op. It's not like the client player gets anything out of them anyway. As soon as my partner and I abandoned the idea of progressing the game, we started having a lot more fun with open-world shenanigans.
Some things do carry over for the joining player, namely money, perk points and unlocked perks, and consumables. So we ended up concentrating on the stuff that had value for both of us, like liberating outposts with plenty of loot in them, or exploring the map to find hidden buildings with interesting lore text to absorb.
I've found it unusually difficult to take a stealth approach with Far Cry 5 as it is, though your experience may vary. I've spoken to some players who've been conquering outposts undetected no problem, whereas for me and many others everything seems to devolve into an escalating firefight within a couple of minutes no matter what you do. There are so many more enemies in the world than in Far Cry 3 or 4 that you're unlikely to stay safe for long, regardless.
Once, my co-op partner and I painstakingly scoped out an outpost and stationed ourselves at opposite ends for a coordinated stealth attack, only for a convoy of armed cultists to randomly drive up the approach and blow the whole thing. With two players who can get spotted instead of one, it makes things even more unpredictable.
Once, we managed to liberate a small outpost with one of us using a silenced sniper rifle on the hills, and the other sneaking around inside with throwing knives. It was almost certainly a fluke. Here's what was more fun: one of us getting in a plane and bombing the shit out of an outpost from the sky before the other one parachuted in with a submachine gun.
THAT disabled the alarms pretty damn effectively, though that was also how we discovered that if you get too far away from each other, the game teleports you back to the same location. I was teleported straight out of the plane... and heard the thing explode as it hit the ground miles away. Driving in with a monster truck kamikaze-style instead solved that particular problem.
You unlock perk points in FC5 by completing various challenges — kills with certain weapons, takedowns, et cetera — and that stuff unlocks in both solo and co-op. Yesterday evening, I started scrolling down the list to see what I still had to do, and saw that although I'd ticked off most of the combat-related challenges, the hunting and fishing challenges were still available.
This led to several happy hours with my partner pootling around in boats and finding fishing spots. I highly recommend this. It's such a chill experience compared to the rest of the game, and as in real life, virtual fishing is much less boring with company.
Unfortunately, when we headed up a mountain to try and find some cougars to hunt, we crossed over into a territory that hadn't been liberated yet, and within five minutes we were being hunted down by two truckloads of cultists.
Once you're past resistance level one in any region, you can't bloody well go anywhere without enemies descending upon you. Once, my partner was kidnapped into a story mission right when we'd gotten into some remote territory to find some grizzlies, and when we finally freed ourselves again we were dropped somewhere miles away on the map. This is exactly as annoying when it happens in co-op as it is when it happens in your own game.
My advice for Far Cry 5 is simple: embrace the chaos when it comes to combat, ignore the story missions, and go huntin' and fishin'. When you stop thinking about all the tasks and side-missions you have to mop up, you find yourself playing the game an entirely different way.