Mass Effect 3 came out in 2011 and to this day, I still jump into some Co-op action. Admittedly, much of that is due to the grind ti takes to unlock some items, but the gameplay is so fun that I'm more than willing to put up with it, and it's one rare case where I prefer this sort of multiplayer unlock system as it gives you motivation to play more. The mix of guns and Mass Effect abilities makes for a fun and challenging co-op experience. You must properly handle the balance of weapons an abilities on your built character. Unfortunately, you can get stuck with a skill build that doesn't work in your favor, so you may have to reset your powers or promote your class after level 20 so you can reselect your abilites.
The grind comes from the unlocks system, which requires you to buy blind boxes which it's loot is determined by the almighty RNG. This is often irritating, when buying boxes and unsuccessfully trying to get a rare weapon, upgrade, or equipment. Even worse, helpful items such as medi-gel to revive yourself and cobra missles to instantly kill groups of enemies can only have it's capacity increased by these rare drops, which are often key to the more rewarding Gold and Platinum difficulty. Generally though, it's a better idea to "grind it out" in Bronze and Silver to get the credits needed to buy better weapons and equipment. Eventually, you will unlock powerful characters which will make Gold and Platinum more easily. Now you can play more aggressively in mission waves so you can help your team win credits, as bonuses are awarded for speed.
Another irritation about the grind and credits system is that the store is a bit sluggish at times. Also, boxes can be purchased with real money with denomination/equivalents from $.49 to about $5 (or $6), just for one box. Either spend alot of time, or spend alot of money and hope you are lucky. I'm not sure if EA/Bioware is making enough money from these "micro-transactions", but if they sold bundles of boxes with these same amounts instead of a single box, I might actually consider spending money to get a few jumps ahead.
Teamwork is key as well. Not only for revivals, but synergy through weaponry and powers, as some classes work well with others thanks to combinations of buffs, debuffs, staggers, and explosive kills. Unlike Tranformers: War for Cybertron's co-op, the experience and credits are shared. So even if you have "that one bad-ass" moping up the room, that's great for the team as the XP will apply to all players and helps tremendously when you're trying to max out a level 15+ character for Gold.
Did I mention all the co-op multiplayer DLC is free, including extra maps, weapons, items, and characters? It unfortunately adds more grind, as you try to unlock these things, but they are often the ways to make Gold or Platinum easy enough to get the credits to buy more upgrades. There still seems to be a reasonable amount of players still playing on PS3 and PC, which is a testament to a good game, despite issues with load times and wave glitches.