How useful is the armor penetration mod?

All the AP mod does is penetrate cover / walls to get to your target instead of having to wait for them to pop up.

So I generally put it on snipers or long range weapons to nail them while they hide.

Shorter range stuff it's probably not as needed as you are close enough to flank that cover one way or another.

Drill rounds. Always drill rounds.

Nope, they removed that feature. All AP mods do is increase how far you can shoot through cover and how much damage it does on the other side.

Which guns did it work with in which didn't?

Fuck me, I got a receiver last night, tried it out on the vanquisher and raptor this morn and was like uhhh... coupled with the loss of built in penetration this is just silly.
*it's fucking nothing!*

It's convenient to just be able to shoot enemies through cover, but that doesn't make it the best option.

I use piercing on slow characters who fight at a decent range because waiting for enemies to show themselves is annoying.

I believe it also works on the Inferno.

It shows up as something you can use to equip the Scorpion ... which I assume is a mistake.

Those weapons that can't take advantage of penetration generally don't have it as an equipable option ... e.g. the Venom and Naladeen.

I thought it was a nice upgrade to the mods, not allowing mods to go on weapons that cannot benefit from them, such as a rechargeable SR not allowing for extra spare ammo.

I would not class the Scorpion mod as a bug as much as a mistake. Perhaps in the next patch, they can fix the Receiver and Smart Choke mod bugs, as well as correct mistakes.