Usually my answer is no. I sign them and then, personally, complete disregard them because I think it's highly unlikely a company would enforce it on a low level employee like myself. The fact that they could try is usually enough to scare smaller employers away from even trying though.

Most sentiment seems to be that they "would never hold up in court". Every once now and then a case does come up though http://arstechnica.com/tech-policy/2014/07/amazon-sues-ex-em...