Some swear by them, others do not.
Wyches are a cheap combat unit with a high survivablity rating against power attacks of all forms, something that's actually pretty rare.
They pack the options to further better against all targets. Haywires for Vehicles, Wych weapons for Infantry and Hekatrix' with power weapon options.
However, on the army level, Kabalite players will argue that the Venom is be far supieror to the Raider. Tournements prove this, as the Venom spam is now one of the most competitive lists out there.
This means that the more streamlined armies tend to pick either Wyches OR Warriors. Each has its benefits and each has its fall downs.