Ring of dexterity
Version 0.17: This article may not be up to date for the latest stable release of Crawl.
A ±X ring of dexterity affects the dexterity of the wearer by X (ranges from +6 to -6). If X is negative, the ring is normally cursed, and it can cause stat zero if you are unable to remove it.
Desirability
Although more dexterity is always nice, the exact impact of boosting your dexterity really depends on your Dodging and Stealth skills, as well as the encumbrance rating of your armour. Evasion-based fighters or sneaky stabbers will likely appreciate it far more than a minotaur in crystal plate armour.
In any case, even a +6 ring of dexterity isn't going to help you enough to make it competitive with many of the other options out there. Definitely use it in place of nothing, but don't feel bad swapping it out when something more crucial comes along.