Never have signed one, never will. I have heard horror stories about them.

I think the greatest fallacy concerning non competes is that they are unenforceable when in fact they are very much enforceable. Not so much in CA and NY but pretty much everywhere else.
This is kind of a timely topic because I recently acquired a stake in a 8a contracting firm and the majority partner is a former corporate counsel for a large environmental management firm. We ahve been going around and around about this. She being the legal eagle wants everybody and their sister to sign a non compete. Me, being the eternal optimist doesn't really see a need for one.
Most non competition contracts are signed at the onset of employment when everything is rosy and they are invoked at the end of employment when things aren't so rosy. A lot of employees will just sign the things because they don't want to rock the boat at the beginning. Some companies bury the thing in the packet of pre employment forms you have to sign. Personally, I don't think I would want an estimator or PM that didn't take the time to read the fine print. That could cause all kinds of problems further down the road.
There is a couple of interesting things about non competes that I found interesting. One, is that they are enforceable even if the employee was laid off and the other is assessing damages on a new employerethat advertent or inadvertently hires an employee that had signed a non compete. Even if the contract is written where it overwhelmingly favors the employer the judge has the right to modify it to make it more equitable which is a little different than the norm.