I run mine with shardnets. Everything everyone else here has said about them is correct. It also feels great to totally shut down a key character in assault by getting a couple of shardnets in base-to-base with them.

Wyches are a tarpit unit in which the Agoniser is the item of wargear that really kills anything. Shardnets enhance the Wyches' tarpitting abilities even further, and is therefore the Wych weapon I've had the most success with.