Is this good or bad?

[Magipi]

the +8 chain mail "Usol" {Acc+5 Dam-6 SInv}

[Sar]

It's kind of awful.

[TheDefiniteArticle]

Dam-6 is ridiculously bad unless you fight with nothing but magic, but in that case you almost certainly don't want a chain mail to begin with. Acc+5 is totally wasted thanks to the Dam-6. See Invis is not particularly good at all, as you can usually swap to it on a ring and even when you can't it's just a convenience in most cases (and doesn't actually do anything against the vast majority of enemies).

It's hard to imagine using it because even for a character ideally suited to it, finding/making something better is reasonably likely.

[Sar]

I guess I would use it if I got it on D:1 and I also had a plutonium sword or something like that. Theoretically.

[crate]

basically this is asking the following question:
which of the following rings is better:
1) the Ring of Robustness {+8 AC}
2) a -5, +6 ring of slaying

I think 1) is better than 2), so I think this chain mail is better than a +0 chain mail. Of course, you have other options that are not just +0 chain mail, but you can easily adjust the rings in this comparison to be appropriate.

[Patashu]

There's a difference between having no +dam and putting on a +6 ring of slaying, vs having no +dam and putting on something that gives -dam.

Points of damage when you already have a lot of damage are more likely to go past AC (if AC roll is higher than your damage roll, no matter how much extra damage you had, all of it was wasted. But as soon as you pass the AC roll, every new point of damage is a guaranteed +1 damage, so +damage bets better the more you have of it).

So -6 damage is a lot 'worse' than +6 damage is 'good', due to the utility of +dam going up the more you have of it.

[crate]

So -6 damage is a lot 'worse' than +6 damage is 'good', due to the utility of +dam going up the more you have of it.

In a majority of situations this doesn't matter. Given everything we know about the OP's character, I think my comparison is the most reasonable one you can make.

[TehDruid]

Throw this at the next lava pit you can find. It won't be missed.

[Magipi]

Throw this at the next lava pit you can find. It won't be missed.

Too late for that, one of my orc knight companions is wearing it already

Thanks for the answers, everyone.

[Klown]

Throw this at the next lava pit you can find. It won't be missed.

Too late for that, one of my orc knight companions is wearing it already

Thanks for the answers, everyone.

If he dies, you know you made the right choice.

[Curio]

When he dies, you know you made the right choice.

fixed that for ya