I signed a non-compete at my last job because like one of the pp, I really needed the job and beggars can't be choosers. However, when it came time to leave, I really agonized over it, and actually paid an employment lawyer for an hour of his time to read it and give his advice.
He said that 99% of time companies never pursue it, but he said ALL of his clients are the 1%, so it can happen and it is devastating. Even if they can't ultimately win and get anything from you, they can file an injunction to keep you from working until the case goes to trial, which could easily be 6 months or more.
My husband stays home with our young children and I am the main breadwinner. The lawyer advised me not to take that chance at this time in my life. I didn't interview with the company that could potentially be construed as a competitor and took a job with another company. I am glad I did, because my conscience was clear and I didn't lose sleep worrying about whether I would get hauled into court.
For me it was a peace of mind thing more than anything.
I signed a non-compete at this job, but it isn't as restrictive as the last one, and I know there are ways to find jobs that wouldn't violate it.