I've searched the forums for builds and the term 'wisdom', and commonly, people seem to take wisdom for priests. Why?

I tested it. It does almost nothing.

The tooltip states, quoted literally: The amount of Mana necessary to cast a spell and the Stamina loss when casting it decrease. The probability of successfully casting a spell increases and the time needed to restore energy after casting it decreases.

From this I assume the following effects:
- Less mana and less stamina for casting spells
- Better chance at hitting targets with spells (the %-chance you see when you fail)
- Able to cast faster after the last spell.

The first is correct. After raising wisdom from 1 to 15, 'Locust Swarm' went from 18 mana cost to 12 mana cost, and from 4 stamina cost to what seems to be close to 0. But this hardly matters after a certain point when your mana and stamina is already quite high and you're already bringing along a bag of potions anyway.

The last two, better chance and faster casts (or faster re-casts?), don't seem to work at all. If I pick a spell that my character isn't good at and attack an enemy, I get 48% with 1 wisdom. After pumping up wisdom to 15, I still get 48% when attacking. Whereas the mana-cost in the spellbook updates according to your wisdom, 'casting time', 'penetration %', and 'rest' numbers don't change at all from 1 to 15 wisdom.

Casting the buff 'insight' on myself, the cooldown period is 15 seconds. After I put 15 points in wisdom, the cooldown is still 15 seconds. You can test it by casting wisdom on yourself and then keep trying immediately afterwards so you can see the time remaining. With or without wisdom makes no difference. This also holds true for combat spells. Edit: and it doesn't seem to influence the 'cast' time either.

Am I missing something? If not and wisdom is actually as useless as I fear, is there anything else that's useful for a priest build? Apart from branching into a bunch of magic-classes, all the priest and melee-specific skills seem close to pointless.

The number of skill points invested into Wisdom is not all that matters. What is your current Mastery Level in Wisdom? According to the rules I have at my disposal, bonuses granted by Wisdom should work like this:

Apprentice: lowers Mana cost by 2X% (where X is the Wisdom skill level)
Disciple: lowers Stamina loss when casting by 1/2(X-5) as well
Journeyman: increases hit probability by 3(X-15)% as well
Master: reduces time needed to restore Mana by 4(X-15)% as well

Check your current Wisdom Mastery Level to see what should apply to your character.

As far as priest's useful skills go (apart from all those various schools of magic which you can all fully explore in one playthrough), it is wise to invest a few points into Melee Combat (to reach the Disciple level (the highest for priests) because you never know when you will need one final melee blow against an advancing enemy while waiting for your Mana to regenerate), a few points in Perception are also useful so that your character can notice hidden stashes and doors, and if you want to mix yourself a few permanent elixirs you can go for the Master level in Alchemy as well. Do not invest in Identification because this skill doesn't work and it is far better to use the Sixth Sense spell. I didn't invest into skills like Holy Aura, Flagellantism, Mana Restoration or Spiritual Insight because I didn't find their descriptions interesting enough to invest my precious skill points into them (I rather maxed out all schools of magic instead).

Thanks! That explains a lot. I wasn't getting up in mastery levels even though I seemed to have all the requirements. It seems that the intelligence I've gained from gear doesn't count towards your mastery level, so I wasn't getting above disciple even though everything looked alright.

However, with some more testing the effect still seems to be pretty negligible. If spell penetration as shown in the spell-book is already at 100%, even if the chance shown in-game when failing to hit an enemy is below 100%, wisdom won't improve this. So when I made certain that I was at journeyman wisdom and tested things with a spell I'm definitely no good at (breath of death) I got 16% before I put points into wisdom, and still 16% after I put points into wisdom. So any spell that starts with 100% spell penetration won't benefit from wisdom other than mana/stam costs.

I also tested it with another spell that wasn't at 100% penetration, confusion, at 60% spell penetration. Casting on a rogue outside the town in act 2, I get shown that I have a 9% chance with wisdom at 1. Then, raising wisdom to journeyman 18, while the spellbook says it now has 65% spell penetration, the chance I get on failure shows 10%, only an increase of 1% hit chance. This might have stronger effects on stronger enemies though, as I assume that spell penetration counters resistances and the enemy I was trying probably didn't have a lot.

Still, before I put points into wisdom, maxing out the magic schools is disproportionately more beneficial. Shield of Faith, for example, goes from lasting around 20 seconds to almost 50 seconds with maxing out Magic of True Faith, and also goes from around 12% magic resistance to 40%. I wish wisdom would decrease the global 'rest' cooldown timer, which is really the most punishing aspect of a caster. Not being able to cast anything for X seconds after the last cast makes a lot of buffs useful only pre-battle while during battle you're far better off casting quickly re-castable damage spells.

I didn't know you could brew your own permanent elixirs with alchemy, I might try that later on as well.

I assume there are some other elements added to the equation when fighting an enemy (like those resistances you mention). There surely is some math behind all of this but only programmers could tell us more.

BTW: When brewing your own elixirs, make sure you use miraculous potions for them.