They are "supposedly" un-enforceable here in the State of California, but having worked here in CA for the last 10+ years in the tech sector, every company has one. Plus I have seen people break them, and let me explain how it plays out.
You join Company A, you sign a non-compete.
You leave Company A, and take a job with Company B.
Company A considers Company B to be similar, and thus determine you have broken your non-compete.
Company A files a law suit against you, and drags you through the courts. This ends up costing you, the employee (my friend blew over $50k in attorney fees fighting it) a lot of money to fight.
Company A also sends a letter to Company B.
Company B let's the employee go to avoid getting tied up in the mess.
So you may say their un-enforceable, what's it matter. The case was ultimately withdrawn (over 18 months later), he had long since lost his job with the Company he had joined, and was out over $50k in attorney fees which he never recovered.
So it doesn't need to be completely enforceable if the company can simply bully you. This is what happened to my friend. Gotta love the State of California, where Company's can use the Court System to basically bankrupt you whether or not they have a case against you.
Interesting. As another California employee, let me tell you how you SHOULD be working it:
Company A hires you and puts a non-compete in front of you.
You say, "This is California and non-compete agreements are meaningless".
Either Company A takes the non-compete away and all is good, or Company A insists that the non-compete is required. If they require you to sign a non-compete it is because they intend to use it as a cudgel. Grab your bag, leave the office, and never turn back.
I currently work for an international company where "every employee" is supposed to sign a non-compete. Except any of us in California, that is. We never signed such an agreement, largely because such an agreement would be completely unenforceable in CA. Had this employer insisted on a non-compete, I would have turned them down and gone to a company who believes it can hold its employees by treating them well rather than with lawsuits.
Really. If you sign a non-compete in CA you're foolish or ignorant or both. You should not ever do it.