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In games of 1500pts is he worth it? or would a regular Warboss and a Big Mek be more effective?
Sure - he's a killer. I've never used him, but I've seen him many times (and fought against him). So yeah, he pulls his weight no matter the size of the game.
Ghazghkull is always worth it, as Cossack said, but in the same way that a unit of nob bikers is always 'worth it'. That doesn't mean he should be used in every single list though. He'll take up a lot of the points in your list, so you have to decide when putting the list together if he's worth it for the army you're using.
The reason Ghaz is worth it is because of the full 6in waaaaaugh he gives the army, his 2+ invulnerable save the turn you waaaaaugh, his immunity to instant death (the only character in the Ork army that has this), and the fact he has all the powers and benefits of a normal warboss in mega armor.
Put simply, Ghaz is the man...I mean Ork. ^_^
dammit...I've got him in a box here. Time to glue him together and add him to the painting table.....(should put me at 2000 points of painting table stuff)
Don't envy the job you have of putting him together. Ghaz is a heavy chunk of pewter, by any standards. ^_^
Good...I'll hold off then so I can drill and pin him...do it right.