Here to raaaaaage about the opinions on ME3CoOp! I kid.
I totally understand the distaste associated with the idea of a Bioware game having multiplayer. I'll echo the first post, the microtransaction element of it was the level worst of any micros I think I've ever seen in a serious AAA game. However, it was by no means necessary. Further, and I think this is why I stuck with it so long and probably isn't an experience that can be applied retroactively, every paid single player expansion was accompanied by a free multiplayer expansion that added new, unique weapons, classes, levels and enemies to the game. The multiplayer received a LOT of dev love and I think at the time that understandably endeared the community to the game.
Plus, for me, it had the perfect mix of rewarding co-op strategy and frequent and abundant sidegrades plus a dose of variable challenge. I don't know that it's easy to tell a person to get into it now, since a ton of the good will was had by being there as the game updated and the devs were responsive, but as the first poster said you can still find pub matches years after development stopped. That says something, I think.