without a doubt the voidwalker is the best demon a warlock has, i use ormgorg (my VW) everyday for questing, farming, instances you name it! 

if i could rename my VW i wouldnt bcuz u grow to like the names of your demons after awhile,
with demon specc'd and soul link  a VW can hold its own for awhile, ANY player that says the VW is useless should be beaten upside the head with a baseball bat =P

VOIDWALKERS RULE!