In my experience, NDA's are different than non-competes. As a development consultant, I am fine signing an NDA that protects the IP of one party, as long as it does not prevent me from providing my services to other parties. If 2 prospects have the same idea, it is my choice if I choose to work on either of them. Doing so does not break the NDA as long as I don't share IP between the clients. NDA's, worded properly are typically not a problem. If a client ask for a non-compete, I will politely decline.
