Skooma is mechanically just a primary amine with an endorphin inhibitor. If you were going to list its behaviors it would get you high for about 6 hours and have a 12-18 hour come off. While not physically addictive it would have a high mental addiction. But more than that Skooma is made from Moon Sugar and Nightshade.
Skooma would contain high levels of Atropine, Solanine, and Nicotine. All stimulants, all poisons. Scopolamine is a sedative effect. I'm going to assume that because a single dose of Skooma doesn't kill you outright Moon Sugar is an adaptogen similar to a Cannabinoid or perhaps plants with an MAOI property like Rhodiola Rosea. Those all give the impression that Skooma itself is much like Methamphetamine so a huge boost in upper brain function but a limiter on Monoamine Oxidation in the brain.
That suggest that you experience no negative effect until the come down, when the drug clears the system. So if you're high all the time you wouldn't experience it. It's a designer drug, and much like MDMA it's not designed to be something you ever stop ingesting.
I'd say that there are no mental defects unless you miss a dose where you literally lose your mind. -2 Int, Wis, Cha. But on the effects you gain +2, Dex, Con, Int. Skooma doesn't boost your strength it is just an amine so it inures your pain response and locks your brain in a state of hyperactivity.
I'm not a chemist mind you, but I know a thing or three about drugs. Enough to know that you need to understand the mechanism of action before you can rate the drug itself.