I've only heard of them and from what I can tell, in just as little detail as you have. But what I did hear has marked simerlaritys with London church of Christ.... apparently the same MO.
I've seen what can happen with a (supposedly

) fully grown adult in that situation. They cut him off from his friends, basicaly took over his social life so they WHERE his social life, so they could dictate who he was able to discuss things with. Cut off access to any ideology but their own....
Luckaly he had enough life experience that he did eventualy re-assert himself... after quite a struggle and 4 years of being in their control, but he did start thinking for himself again. (I still think he is an idiot for letting himself get drawn into that situation in the first place!)
The 'scary' part about groups like the brethren, is the whole school thing. Indoctrinating kids before they get any life experience so the only way they know how to think is the way the group wants them to think. (Hence not wanting them to go off to collage or university where they might be exposed to other mind-sets and actualy start to think for themselves.
Groups of that Ilk I find hard to apply the lable 'religious' too.... because the base line isn't about Theology at all, that is simply a tool used to achieve the true objective..... power and control over others.
Fenris