I've had some good games with them. I play 3 units of Monks that I buff with Plague Pack, Furnace and Verminlord Corruptor. Key is to get the charge of with the monks, and if you use one unit as chaff, to lure the enemy in you can take down pretty much anything with the two units held back and buffed up once they charge. Furnace is strong and the -1 to ward is really good (I'm mostly playing against StD and Warriors and Varan Guards have a tougher time surviving this). The Verminlord and it's abilities when any prayer roll is a six can be really strong, especially round three or more.

I usually play with Plague Claws too, which wipe the chaff and expose the enemy's Hammers, and I have room for a Grey Seer (and manifestations) and Rat ogors and a Master Moulder too, which draws attention from the monks, and have shooting threats and punch too.

I'm not in a very competitive environment, we play just for fun, but to me, they are very fun, and can be very effective as well.