I've got a set of them, the wizard on disc, and archaon. The formations are good, but the varanguard are simply amazing by themselves. They can take a beating and have awesome weapons.
Their ability to attack and then pile in and attack again once per game is ridiculously awesome. Esp if you take the -1 rend, 6 attacks each weapons, and then stay near archaon (reroll 1s). Keep them within 18 of archaon, and use archaon of a wizard to buff their save and they are a tank!
They are my favorite chaos unit