I think they are illegal in California, a state that protects employees' rights to earn a living. You should look at the specific wording of yours. To be valid in states other than CA, there must have been consideration (something) given in exchange for the agreement. There must be limited duration, limited geographic area. Your state law might even have something that says it's illegal if it creates undue hardship. Your state probably has an employment law section on its website that talks about this. If you decide to talk to an attorney about this, you want to ask if he usually represents employees or represents employers.