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I, for one, absolutely adore them and would take between 5 and 10 units. But, then again, I'm insane and find them ever so useful. You can use them to get up there and be flankers, if your enemy has long range it just means that they will have that many more things to shoot at (and distract them). They can be used to take out pesky long range enemys (though... Fell Bats are better at that). There are a few million more things that they can do, but I just want to hear your oppinions.
I find them useful, but within limits. I like two units of six or seven if I have the points. Otherwise, I might take one unit of eight to ten. I find that they need numbers so that they can suffer a bit of atrophy before reaching combat. They go down so easily to missile fire, magic, and god forbid that they have to take a charge. They're such lightweights.
If they can get a flank, great. If they can make it to the warmachine, great. I find them excellent harrassers. But I'm reluctant to field a whole lot of them. And I was buying them in an effort to run a Sylvania list, but once I learned in a GW FAQ that you must summon at least 5 of them with wolf lord invocation of nehek, just as you would zombies or skeletons, I thought, "Well, that sucks when you're only rolling D3, 2D3, or 3D3." It's not worth the trouble.
Yeah I use them, they probably aren't the most effective unit in my army but I'll still take them. They provide an annoyance if nothing else, and can be very useful when left alone. Also they provide a perfect spot for my fast Carstein thrall to hang out.
I like them, but you have to keep them away from archers and the like. They're great for running down stuff, but a lot of undead players can't use them effectivly enough for thier play style so they shun them off. Like every unit in the game it depends on your play style really.