As always, listen to what Stitch says.
The EC is pure HQ ownage.(
) In single combat, he can take down most HQ units, with the exception of tooled up Chaos, Nids, and a few others. If attached to a Sword Brethren squad or even a CC Terminator squad, nothing short of a demolisher cannon can stop him.
In my list, I take an EC, a Marshal, and MoS. I still haven't posted it up though, so how points effective that is is yet to be determined!