If we had constant monitoring by face recognition systems, I don't think I would mind. If it can make things simpler, like going through the airport, then I'm all for it. I don't think it would be as invasive, in terms of privacy, as some other systems may be.
I feel that it really depends on the type of criminal that should be identified by a face recognition system. I think the most important criminals that could be detained by face recognition system would be criminals that are on the run, such as people who have just robbed a house or a convenience store. That way, if they try to flee to a busy place to be hidden by a crowd, they could be apprehended.
I'd be really mad if I were wrongly detained because of a mistake made by the face recognition system. The system should be fool-proof enough not to make those mistakes. I could understand a mistake here and there, but not a couple every day.