Unless someone has had direct experience with an NDA, I doubt anyone besides a co-op/condo attorney has the background and knowledge to properly advise. I worked for a very large IT corporation and we were not allowed to sign *any* NDA without in-house counsel review. Much better to err on the side of paranoia... :-)

But to answer your question, I don't think they are intrinsically illegal. For example, suppose a solar energy company wants to use your building as a test-bed for a new type of solar panel. They may ask you to sign an NDA preventing you from revealing facts about efficiency, problems, cost, anything like that. In my opinion, that sort of NDA should be ok.

On the other hand, suppose one of your shareholders has some adverse knowledge about another shareholder that they won't divulge to you without a form of an NDA. The first person you should go to is your attorney. Otherwise, you may end up liable for things (and financial penalties!) you never dreamed of. Ignorance of the law is not a defense.

Once again, this is just my very non-legal opinion and you should not rely on it as any sort of legal advise. If someone is asking you to sign an NDA, speak to your lawyer.