If you get the rule with rending they can be devastating if coupled with someone like Yarrick who allows rerolls to hit on charge.
I was able to charge a friends necron overlord and chop him to pieces in 1 turn with no hassles. I would only use them in this fashion.
I guess they would also be worth a shot against space marines where the armour saves will usually save their backsides. Just need to hope for a 5 or 6 when rolling what they are
if you get 1 or 2 then they kind of suck .