Usefulness

So just how useful is Poison?

Very. The damage done is proportional, visually about 20-40% of hp remaining. On harder bosses and Ancients it can take off several hundred to several thousand hp off a full hp bar. I use it all the time.

It's only useful if whatever you're trying to poison can actually get poisoned, and has either high hp or more than 50% hp left. Poison can't kill, but it does take a percentage out of their current hp (so poisoning something that has only 1/4 of its hp left could be a bit useless depending on the enemy)

What about on Bandits? I mean the Vets and Masters.

You can poison certain field bosses after some effort. Their HP loss is really useful from the poison and makes them easier to take down.

I use poison all the time with Master Bandits; e.g I'm taking on 2 bandits while my snake poisons the other two.

Also remember that weapons with poison lathered on it isn't as effective as having a snake with Poison attack.

though I'm not quite sure about the venom sword...

I can see the logic behind poison-lathered weapons and snake venom...

But just how strong is poison-lathered weapon compared to snake venom?

I know it's less effective...as far as values go, I have no idea.

Poison is poison (though I have no proof in terms of pet vs pot/bottle), but the fact that you don't have to frequently lace your pet with poison makes using a pet snake more practical. That being said, that snake's ability to poison won't be of use to anyone if it's dead :x Either pray that you poison the target before the poison wears off on your weapon or hope that the target doesn't kill your snake before it gets poisoned.

Out of some 1000 bottles, none of the weapons I have poisoned (Mainly swords...) did not poison after the very few uses it had before the poison wore off from the weapon.. As for the snake, after about 30 feathers, it would finally poison the Giant Long Gnu. :x I could be unlucky...or maybe some had more luck than others. Maybe there's poison resistance?

I dunno...maybe poison is poison, regardless of origin. I find snakes more reliable though.

The probability of poisoning is based on the cp difference, so you probably (definitely) had higher cp than your snake.

Which goes back to what I was saying earlier about not having to constantly lather your pets in poison (though that would be cool). I tend to train my pets to around lvl 50+, so that accounts for why my snake has a better chance at poisoning whenever I'm on my alts. The venom sword is a more expensive yet effective alternative to snakes and poison lathering, but not everyone has one (anymore).

Edit: forgot to mention that although you technically have unlimited "hits" using a snake (until it either gets killed or runs out of summoning time), poisoned weapons do have 2 huge advantages: you can use them any time (can't summon mr./mrs. Snake while pulling that cart around), and splash/AoE skills like windmill, crash shot, r5+ smash/magnum shot can technically poison everything that gets hit, and it would only count as 1 use. I've tested this out while running sulfur spider. Tl;dr use a snake for 1 opponent, and a combination of a poisoned weapon and an AoE skill (That isn't magic or alch) for multiple enemies.

Well then this raises the Question: Just how useful are Venom Swords?

Using venom swords comes with the same risks as using regular melee skills. Although the base damage range and crit rate are nice, you'll still have run up to the target (vs using a snake as cannon fodder that can poison). The whole point of the venom sword is to deal decent damage, while at the same time, constantly having a chance to poison whatever it is you pulled the sword on.