(( Hey Warlord,
From when I've seen them used, and had them used against me, they were using MSUs. They only get worse, when you have multiple units of them.
Locally, they've been restricted now to 1 unit before penalties. But, being as it is a house rule, enforcement is non-existent. But... now I'm starting to see 1 unit of 8 - 10 models, instead of 2 - 3 units of 5 models.
Anyway, to echo what's been said before, they really are utterly ridiculous. They're a no-brainer unit. And, as far as my experience has been with them, there is really no effective way to deal with them. They cast spells cheaply, the 4+ Ward Save makes them highly resilient to a ton of stuff, and they have the mobility to pretty much go where-ever they want too. If you do catch them, they still put out enough hurt to be forcing break tests.
Oh! Thinking of that, they also channel since they're wizards.
About the only semi-effective ways I've seen of slowing them down is to Curse of Ahnrier or Uisirian them, though they still get the 4+ ward save, so it only meagerly slows them down.
Though... they are pretty expensive to buy, the models look pretty meh, and have some of the fugliest horses I've ever seen. ))
