This is what I've heard (at least in Pennsylvania where I work, and from a number of family members and friends working in different HR departments.) IANAL.

Non-competes are unenforceable because you cannot take away someone's right to work. I'm talking even about switching to your previous employer's competitor. Say you're a software engineer with a lot of domain knowledge on heavy construction. And that employer lets you go (for whatever reason). If there's one other heavy construction company within driving distance hiring software engineers, neither the state nor your previous employer can keep you from using your talents to make a living for yourself.

The only exception is in NDA's. You can't take proprietary information with you, whether it's source code or sales leads.