I think you first have to judge Heavy Mattock against the other human specific equipment out there (Silver-Inlaid Dagger, Sharpened Pitchfork, and Butcher's Cleaver). And when judging everything put together, include effect, converted mana cost, and equip cost, I think this is the worst of the four equipment pieces and I rank them in this order:
- Butcher's Cleaver
- Silver-Inlaid Dagger
- Sharpened Pitchfork
- Heavy Mattock
Heavy Mattock wants to be in Human Decks. Put it in a Human Deck.