I love Mexico and its people. But, I also recognize that the saying "Good fences make good neighbors" is a truism. The US government has long pursued a course of token border enforcement, apparently under the theory (perhaps proven true?) that cheap labor will make us strong. Regardless, laws should exist and be enforced, or should be modified and used, or should be eliminated if we continue not exercise them. We have quasi-legitimized illegal immigration to a degree which can't possibly be good as a long term solution, by giving people here illegally greater freedoms and legitimate access while stymieing the ability of police to act when they know a person is here illegally.
Try as I might I can't find anything unreasonable in body of the Arizona law (as updated). No one I've heard debate this issue has pointed to anything within the law they find flawed; they have only expressed grave fears the law will be used in a manner which is unreasonable and/or illegal. And that is a very legitimate concern, and one larger than Arizona and this particular issue. It is something which can and should be addressed nationally, so that all people are protected. Checks and balances must always exist to prevent the abuse of power, in this case racial profiling. To this end, all police officer's activity should be monitored and tracked. Any time a traffic stop is made the ethnicity and circumstances should be recorded for statistical purposes. Periodic checks should be made of video from traffic stops and other police-civilian interactions to ensure that the police are accurately recording their statistics. Through these statistics it can be understood whether police are behaving appropriately, and adjustments can be made to ensure they do. It is short-sighted to focus only on objecting to a reasonable law simply because you fear it will be used unreasonably. We are a nation of states, and the purpose of having separate states with state powers is to let each state be a petri dish to culture new cures for our collective societal and governmental ills. We must allow individual states the chance and freedom to develop individual solutions, especially when the federal government is unwilling to fully recognize the patient is sick. Clearly there are moments when the federal government must a state's conscious and enforce a more enlightened rule of law, but until it is proven that this new law is being abused, now is not the time.
I'm all for legal immigration. I believe illegal immigrants have contributed more than hurt this nation. And I'm all for granting amnesty to all the illegal immigrants already here, but that only makes sense in the context of a country with sealed borders. And that's not something the federal government has seemed to pursue seriously. I have talked to too many people who have described how "easy" it was to get into the US. (I say easy in the sense that it cost them only a few thousand dollars and they weren't caught; the actual journey with the "cayotes" sounds like a miserable and perilous few days.)