Laws like this make it less likely that illegal immigrants who witness real crimes will come forward and talk to police and it also makes them easy prey for real criminals but I doubt the Republicans who passed this give a shit about that as long as they can pander to voters with a line about how tough they are on border security.
If I were an immigration attorney I would be getting ready to file a class action lawsuit for anyone who has or will be affected by this law and I would be prepared to take it all the way to the Supreme Court of the United States where I believe, and hope, it will eventually be struck down as unconstitutional, the issue being the matter of probable cause, and the question being, what probable cause does an officer have to stop and ask someone for proof of citizenship paperwork? The simple answer is none, they have no probable cause, unless, like the idiot Republicans in the Arizona legislature you believe that brown skin is probable cause. Newsflash, it is not and never will be.
My verdict:
Bad law and untilmately unconstitutional.