Another US-based answer:
We talked about non-competes in my one-semester of business law class (I'm an MBA, not a lawyer). I remember the professor (who was a practicing lawyer) saying that a well-written non-compete is absolutely enforceable and not to let anyone convince you otherwise.
He didn't get into the gritty details for a bunch of dumb managers like me, but I remember him saying that it had to be reasonably time bound (one-year was the most common example used in class), narrowly scoped to protect the valid interests of the employer (e.g., you can't prevent someone from working in marketing for one year, or prevent someone from working in NYC for a year, but you can prevent someone from working in marketing in NYC for a year if that's a market your firm operates in), and one other criterion I can't remember any more.
The bottom line that I took away (which I think was all he was trying to get us to remember) is that if you are asked to sign a non-compete, take it seriously.