As a player, I have to say I am not a fan of the stamina system. I originally had high hopes for it, but once I saw it in practice, well... I used to enjoy leveling skills. I found it sort of relaxing. It's mostly been commitments outside of PS keeping me from the game, but due to stamina, the truth is that I have no intention of leveling again once I have the time.
My main issue is with mental stamina (though I don't like the physical stamina either). The problem is, as you level you cast spells more quickly. And the quicker you cast spells, the quicker you lose stamina, so that once you get into the high levels you have to pause so frequently that you are left thinking there was never any point in leveling at all. I don't feel it's realistic that a mage who has increased their skill to cast faster would need more breaks in the same amount of time as a result and I feel, in the very least, the stamina cost of spells should be reduced as you gain levels.
As Rigwyn has observed, r5 spells (like animate shadow) drain stamina very quickly. Lumi has observed this even at r3. (From my previous experiments, up to r2 seems tolerable and then it quickly gets ughy.)
I have nothing against stamina as a concept, but I currently feel the effects of it are much too intense. It interferes too much with enjoyment.
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As a dev, all I can say is that there is still discussion going on but no guarantee of changes. It'll probably get more attention after the next release, and hearing more people's opinions would definitely be helpful, be those opinions for or against the way stamina currently works.