Unless your barbarian need to have giant's instinct for some lore reason or you plan to spam some "large" AoE melee attacks I usually don't recommend such instinct. The +2 damage when compared to a dragon's instinct usually don't compensate the additional clumsy -1 to AC.
Yeah, the best way to play a giant barbarian is actually to poke from afar at range and/or skirmish. Unfortunately that's not entirely rh fantasy people have when they pick the haedest hitting build in the game.
Good part of Giant Instinct is that the Clumsy 1 from Titan Mauler (use a large weapon) does not stack with the Clumsy 1 from Giant's Statute, meaning there is no downside for using the ability. Also a ton of damage per hit.