I'm fascinated to see that there are 3 different threads currently active about OC. For those with an interest, check out opencarry.org.
The 'element of surprise' argument does have validity. So does the 'political statement' argument. The one argument that rarely gets mentioned (and my personal reason for OC) is that it's much more comfortable. Strong side OWB is much better than strong side IWB, and Cross Draw totally beats both. But Cross Draw with a big gun and even a small spare tire simply does not conceal under anything less than arctic Carhartts.
As for the public's reaction, I've OC'd a fair bit in Colorado and Alaska, and the positive comments have out-weighed the negative by at least 10-1. I even inspired a guy to go buy his first gun, and took him to the range a few times and gave him some pointers. So the political statement IS effective.