I would not mind constant monitoring by a facial recognition system. As long as it didn't take more than a few seconds. The reason for this is that it can provide an easier way to check a persons ID, It could potentially help catch criminals, and it could limit the ability to steal a persons identity. Identity theft is a huge problem today even with all of the security questions in place for most things. A facial scan at a store or a bank could prevent people from stealing, or using, thousands of dollars.
I believe that all known criminals should have to be entered into the facial scan system. This could prevent sexual predators from entering schools, day cares, and other places that children gather on a daily basis. It could also prevent known drug dealers from purchasing anything from a pharmacy in bulk. It's also a way to quickly put out an alert for any criminal they suspect has repeated their offense. If you don't want to be entered in to the system, don't get arrested.
If I was improperly detained I wouldn't have a problem depending on the manner in which I was detained. If I was charged and tackled by an officer without even a warning, then I would probably be mad. If the officer Asked me to step aside so he could compare my face to a picture and check my ID card, I would understand. The way that I picture the system working is that there is a silent alarm that alerts the officer or security guard. I think that is the best option, and the will create the smallest amount of issues.