I just read the game allows this on weapons.

Damn, I am so clueless. I figured they were for reverse pickpocketing only. But this is actually good timing, since I'm formulating a new game.

Are they effective?

Do they kill with single use? (I'd like to avoid such power if they do)

This may change my plans on using melee more often.

Also, I'd like to hear some ways you've used them, if you've time to type up a good story or two.

Thanks.

I don't use poisons much. I did on a tribal character and have experimented a bit with others.
Poisons don't usually kill in a single use. They cause damage over time. The weaker ones aren't all that effective as they take a while to actually kill the enemy if they do at all. A strong poison can cause serious damage. They are best used with thrown weapons as they hurt an enemy as they run toward you. On say, a machete, they aren't that great as you'll probably kill the enemy yourself before the poison has time to really work its magic. It is most useful on tough enemies like Deathclaws. It helps whittle away their health while you use conventional weapons.