I personally don't understand what all the fuss is about. To me it's simple, if you're an illegal alien in any country, you're breaking the law to begin with. I do understand that the main concern is discrimination with racial/ethnic profiling by authorities as a possible result, which I would find very upsetting, but I say let's deal with it if that actually does happen.
Prince William County in Virginia says they did it and it's working just fine, there is no racial profiling.