I think basic followers are good to go now. A follower will follow a target (be it a player, NPC or any object) when not fighting. Once fighting, they work like a normal NPC.
If a follower has the NO_DEATH flag set, it will play the death animation when hitting 0 HP, then fully recover after 15 seconds similar to how players do. If the NO_DEATH flag is not set, the follower will die like a regular monster.
If the follower loses LOS or the player climbs a tall ladder, the follower will teleport to the player once the player reaches a viable spot.
Next up on my to do list is scripting the quest. There are 2 occasions in the upcoming quests where I intend on using a follower or followers, but I won't spoil anything here.
In the mean time, go check out the JasX group. I've put a spanksgiving gift in one of the notices that you can use on your coop partner!