Just as a heads-up, taking the Purity perk in the Alchemy tree reduces the market value of potions and poisons that previously had mixed effects.

yup. market value is done poorly IMO since they just sum the values of individual effects, but it would be a bit complex I guess to reduce the value of potions or poisons for having counter effects.

I dont even understand what this perk does... Care to explain while you're here?

Alright, you mix two ingredients. Both have Restore Health, and Damage Stamina. If you don't have the perk, the potion will have both effects. If you do have the perk, the potion will have the more powerful effect. So, instead of being a potion that heals you, but damages your stamina, it will just heal you. Or, if the damage effect was stronger, it will no longer heal the enemy when it damages their stamina.

When you mix a potion it may for example restore health, give resists and reduce stamina. The perk will remove the penalty of reduced stamina. Similarly for poisons you might have a potion that deals damage to health and stamina but say restores magicka. The perk will remove the magicka restore.