There are two main ways to use them; one as flankers, and one as heavy cavalry.
As flankers, you want a unit of 5 with no upgrade. Their job is to get out on the flanks like fer's ones and hit the flank as your main line hits the front. This will negate ranks, give you +1 for flank charge, and give you some strong kills from the flank. Not many units can survive a combined charge, so expect it to be relatively tough to get both to charge in the same turn against a decent opponent. Hit with the front unit, who should hold at least, then in that turn of charge more the CO's to the flank ready for a charge next turn, or to possibly block their flankers.
Secondly, as heavy cavalry. You want a frontage of 6, if you can afford the points then 1 or 2 in a second rank just to absorb the shooting that will come your way, and full command plus either the warbanner or Haunchi's, these guys charge directly at the front of enemy units with your main battle line. They should smash any average infantry, against tougher infantry like Dwarfs and chaos get some support ready because if they don't break in the first turn, your unit will be in a bit of trouble. Though they are also tough enough and strong enough to survive fairly well in a prolonged combat on their own.
