Federal agents have no lawful authority to enforce any statute or other rule in any fashion which would contradict the supreme Law of the Land. If one accepts as a premise that the Constitution of the United States is the supreme Law of the Land, then by definition any action contrary to it is illegitimate. While I am happy to see states declaring their non-acceptance of federal statutes, I'm unhappy with the fact that states' legislation doesn't declare that unconstitutional statutes and other rules are not laws, and do not "become" illegitimate as a consequence of court rulings, but rather are illegitimate from their inception as a result of their contradiction with the Constitution [with the rare exception of laws voided by Constitutional amendments; their illegitimacy would stem from the date of the amendment's ratification].
The problem is that it is the federal judiciary(US District Courts, Circuit Court of Appeals and the SCOTUS) that determine whether federal laws or even state laws conflict with the United States Constitution). While a state may have a legitimate argument that a federal law is contrary to the powers delegated to Congress in the Constitution the state in of itself has no lawful power to nullify the federal law or make a state law in contrary to the federal law. The combined power of a state governor, state legislature, state supreme court and local law enforcement cannot stop the enforcement of a federal law that is on the books by federal officers enforcing the federal law.
The only way this country will return to what it should be is at the ballot box and to elect a conservative President who will make conservative appointments to the federal judiciary. Also, we need a conservative Congress. All of these nullification laws that are only really symbolic is only going to make the federal government stronger in the end as the federal judiciary will likely utterly reject these state laws and establish even stronger precedent against state nullification laws.