Gargantuan gets fragile by the time you reach Hell, specially the later acts. I found out the 20% damage reduction passive to be pretty much required. Sometime during Hell you'll acquire the rune that allows the Gargantuan to stun: this is very, very useful, even bosses get stunned. By act 4 I was resummonning quite often, but still manageable. Gargantuan is useless in inferno.
For everything that isn't as manacheap as the Blowdart you will probably have mana troubles. Any build based on costly spells like Acid Cloud, the Undead Bears, etc... pretty much mandates VQ. Personally, Act 1 Inferno (butcher runs), as my gear improved I recently found out I make enough of a dent on enemies with Dire Bat I can comfortably use it again. SO glad I have an alternative to the blowdart lol.
Spirit Walk, well... someone got to it before me. Because of this, I no longer use Soul Harvest.