Here is a perspective from Megan McArdle:
I'd be a lot more sympathetic to this law, in fact, if it required the police to check the immigration status of every single person they pulled over, without any gauzy "reason to believe" fig leaf to cover up what's really going on.
Raise your hand if you think that law could have passed in Arizona.
Now, anyone whose hand is raised, contact your psychiatrist immediately. You need to check the dosage on those meds.
If you think that immigration is a pressing problem, then the place to enforce it is in areas of life that are already regulated pretty intrusively: border crossings, employment, landlord/tenant relations. These are places where enforcement can be stepped up quite dramatically without massive intrusion into the ordinary lives of law-abiding citizens. But quasi-criminalizing looking different . . . well, it's not just wrong. It's un-American.