Contracts are always enforceable.

There are some provisions that some courts will refuse to enforce. E.g., there's a "strong public policy of the State of California" against most non-competes, and judges will simply disregard that portion of any offending contract.

http://lawzilla.com/content/noncompete.shtml

I am responding to the blanket claim that the contract is unenforceable because CA has at-will employment. That there are specific exceptions to the general rule is not important re above.