On the surface, this seems like a bad deal, at least to me. I've intentionally not used them because of their negative side effect. Besides, a Super Mana potion will restore 1800-3000 mana instantly -- so why wait for the mana and incur a spell penalty?
Phaelia over at Resto4Life recently did some math that may or may not change your mind about using them, if you use them at the right time.
Mostly the article is aimed at discussing when healers should pop a Fel Mana potion, but good news for all you hunters out there that may not have thought of this yet... there's no downside if you use them!