Plane Sailing said:
"Mettle" was introduced by someone not thinking clearly in a splatbook, who said "hey, why don't I have something like evasion, but for other saving throws?" without thinking it through.
This sounds like you are assuming it is too strong and should not have been made

But as you said the same earlier effectively it wasnt hard to guess.
Effectively this ring would partially protect the wearer from 15 spells about.
There are other spells which protect against some/most of them even. But then I have known people who ban evasion because they feel it is too strong and not thought out.
Plane Sailing said:
Basically reflex (partial) saves might be much more common, but there are many more defences against them too. The Fort (partial) and Will (partial) spells were designed in a ruleset where there is nothing to remove the partial effect.
This is true, but several of those ways would protect against spells I listed before as well (such as the shouts for example).
It is very interesting to see where the balance really resides though.
On one had you have an ability to fully resist nearly every damaging spell/power in the game.
On the other hand you have the ability to ignore a handful of spells whose secondary effects happen to fall into a certain category.
The first can be overcome through other means (resistance, immunity, etc)
The second has similar things (spell turning, high tough ac, blanket immunity, etc)
The first seems to be benign enough that there is an improved version.
The second has no such parallel, but then it would be nearly impossible to draw one, because those things that it works on generally have a massive primary effect and then a secondary tagged on almost as an afterthought. Most of the secondary effects are fairly minor, there are about 2 exceptions.
I am still undecided

it is a rough call really. I dont want to price it too high because I want it to actually be able to be used, and since its uses are limited (and they are limited, very) then it is more difficult to actually be shown to be useful. Cant price it too low though, otherwise everyone would have one.. but then, even if everyone did have one being able to save for none on about 15 spells or so seems pretty minor.. very tough call.