This is one of those
NetHack potions that players tend to
hoard carefully, despite its near-total
uselessness ("Oh gee, monster detection? That sounds good. I'd better keep this in case I need to detect monsters someday"). Seriously,
don't even bother.
As far as the potion's effects go, only the blessed potion has any real utility. Quaffing it will allow you to know the type and location of all monsters on the level for 21 to 60 turns. This functions as temporary intrinsic telepathy, only stronger.
The uncursed version only shows you the monsters for a few moments, which can reveal certain monsters waiting in ambush such as a nest full of cockatrices or the dreaded electric eel. The cursed version does the same thing, but all monsters on the level that are paralyzed or asleep are freed and/or woken. This can be a highly insidious effect depending on how you try to use the potion.
Use of this potion displays no messages, unless you are already able to detect monsters, in which case you will "have a strange feeling for a moment."
Yes, I realize this is a stupid writeup, but there's really not much I can say about this potion. I'm noding it for the sake of completeness.