Ring of protection
Version 0.15: This article may not be up to date for the latest stable release of Crawl.
A ±X ring of protection affects the AC of the wearer by X. If X is negative, the ring is normally cursed. Even if you've identified rings of protection in general, you won't know the specific amount of protection a given ring offers until after you've put it on.
Desirability
Depends on the strength of the ring. All characters are going to want more AC, but sacrificing a ring slot for it may or may not be a good idea. A +1 ring is something you'll likely only use until a decent alternative comes around, but a +4-6 ring may be worth wearing through to the end of the game.