Having been playing ADOM on and off for well over 6 years now, almost exclusively as thieves and weaponsmiths, I've come to learn a lot about appraising. And everywhere I've been to ADOM-related, appraising has generally been maligned or ignored. Well, no more.
Appraising turns out to be very useful, but sadly misunderstood. Players expect appraising to at least tell them if an item is cursed, but they find themselves disappointed. But that's not the point of the skill!
The point is to slowly pseudo-identify your unidentified items. Letting you know, passively, what items are total junk and what's either blessed or has unusual bonuses (it even seems to take into account positive and negative ego mods correctly!).
A common complaint is wands of wishing appraise "mediocre". Appraising, sadly, doesn't deal with wands unless they have an extremely high number of charges (~10). However, it _does_ work with all other items. Even though it doesn't work with wands, I find a lucky "good" appraise on that unID'd blessed scroll of identify works well for emergency life-saving measures
But, what about all those good-appraised cursed items? I've found this very rare, albeit noticeable to some extent in every game. This happens when weapons and armor have not one, but two bonuses (e.g. an armor having +3 to PV above the default alongside some other bonus, or a weapon having both a hit and damage bonus).
The two positive numerical bonuses (or a combo of a numerical bonus and positive ego mod, or even two ego's!) counteract being cursed to result in a "good" appraisal. As long as one is aware of this, simply not equipping the "good" items that wouldn't be a improvement is fairly trivial.
I've found the skill incredibly useful overall, particularly early-game when your largest stack of unID'd scrolls (identify is pretty typical) has a single outlier of that type appraised "good", telling you it's blessed, or even more obvious for pre-ID'd watery potions. Alongside my typical use of periodically discarding mediocre weapons/armor to free up carrying room, saving having to burn a scroll of ID to not risk dropping anything useful.
For all those attempting one of the harder classes that start with appraising, hope this is informative. I've found on all of my (inevitably heir-less) thieves, I could use all the help I can get!