To make the best use of this spell, I figure you need two copies of them, ( thus to keep SBP's down, best used by a Wizard ) and a Battle Forge.
Let the BF cast it, costing your mage only 2 net mana, gain 1 mana off it the following 5 turns, thus net gain 3 mana,
leave one counter on it so it's not destroyed, then let the BF bring out your second Amulet and put the first back in your Spellbook ... repeat ...
So, you can gain more than 6 mana out of 1 Wispwillow Amulet and it gives your BF something to do all game long.
I think I'll have to build me a Wizard book next