It ain't ogre till its ogre?
Skullcrushers are 8 RHD giants. The four HD over their non-crushing cousins are made up for by a +4 strength bonus, better mental stats, free weapon and armor proficiencies, rock throwing, and Improved Grapple and Two-Weapon Fighting as bonus feats. Also interestingly enough the natural armor is worse than the default ogre's.
Does this justify missing out on four class levels? I'd be inclined to answer 'no'. A regular ogre is already making up most of the difference in damage with the extra power attack it can use, the skullcrusher's bonus feats are so-so, and the weapon proficiencies are something any self-respecting melee warrior would already have.
If a few of the RHD had been removed, I'd give skullcrusher ogres a reasonable +0. As they are now, -0 seems to be the best assignment.