I only recently discovered that Shadows potions eat up some mana while you're invisible. It's still worth it though usually. Minor/major/super poisons can be great to use for trading, especially since you can buy a small flask for 10, fill it, and resell it for 100 (or a super for 100 and resell it for 1000). I've found that there's plenty of NPCs and merchants who always give you full price for items (including potions) no matter how much they like you.
Also, if you're facing an enemy that uses devastating magic attacks but not many other attacks (say like something that shoots random elemental spells at you) it's pretty inexpensive to mix up and drink 10-12 minor elixer potions in a row. You're pretty much invunerable to magic attacks, and the effect lasts a pretty long while.
