I haven't seen a post regarding this and please direct me if this has been discussed.
I've noticed when giving followers/companions an enchanted weapon, after many battles the enchantment charge was not dwindled. This is perfectly fine with me because I wish to hoard all of my filled soul gems for future enchanting and recharging of my own equipment but my question is, do they yeild the desired effect when using them?
I have not seen anything indicating this (ie. Enemy set aflame when my follower attacked an enemy first) so my curiosity is teeming. And a side note, what would you say is the most effective weapon enchantment? Not far into enchantments yet but so far mine would be the (silent?)moon damage one since I spend most of my days in town and dungeon crawl at night thus requiring less recharge.

I would try giving your npc an empty soul gem and a weapon with a soul trap enchantment, and let em solo something. I'm quit partial to the silent moon enchantment myself, and any sort of soul trap enchantment

Great idea I will certainly try that. Thanks!

Another pretty straight forward enchant would be dual enchant of fire and lightning for maximum damage, or life and stamina absorb for maximum durability.

I also wonder how poison works with companions... like if you apply poison to a weapon and give it to the companion, does the poison last forever?

That would be awesome. I never use poisons myself because I forget to mostly but does the weapon show poision applied in the items screen? That'd be the only way to know I guess.

it's my understanding that spell cost reduction also reduces weapon cost. so if you had 100% reduce destruction, the fire||shock||frost damage on your weapons would also be infinite. after killing hundreds of enemies, my weapon is still at full charge. this mechanism does NOT work on followers. so my follower has 100% reduction and her enchantments have long since drained.

Ah, another new piece of information. Thank you for this.

i gave lydia my ax of whiterun (fire damage) and it lit people on fire

That's good to know. But I have still yet to see my followers active enchanted weapon drain at all. Maybe they are just that terrible ( I stick with archers to avoid slashing/burning them during fights).

According to almost everyone on this forum that's because she's a beast. ( I murdered her in a fit of rage so I wouldn't know : >)

Faendal had one of my staffs, and it seemed to last. They have infinite arrows, too. You can also get them to pick some locks. Faendal opened a master-level chest with no problem after I'd used up almost all my picks.

First thing to check - is your Follower even using the Enchanted weapon? I ask this because when you give them new armour they will only equip something that is a different material to their default set - a Follower wearing Steel Armour will not equip Legendary Steel armour, but will equip Fine Dwarven. While highly unlikely, it's possible that you gave your Follower, that used a Steel Sword, a Steel Sword with an Enchantment that is going unrecognised.

Followers certainly do use enchanted weapons and the weapons certainly do need recharging. Have a look in the inventory to see what the weapon looks like (it will have a certain glow to it) and then have a look to see if the weapon they are using in combat is the same. If they are wielding the correct weapon in battle but you are not seeing any effects, like fire, it's possible that you have a display bug, either in displaying the appropriate effect or in the recharge bar and the weapon is actually out of charges. Have you tried to recharge the weapon in question?

Followers don't seem to use Posions (but they will use Healing Potions).