State's rights took a huge hit with the Supreme Court's decision on medical marijuana. The 6-3 decision basically said that even though the marijuana was home grown, distributed locally, and didn't cross state lines, it can still be federally regulated by the Interstate Commerce Clause of the Constitution. No matter which side of the issue you agree with, Justice Thomas summed it up beautifully in his dissent:

Respondents Diane Monson and Angel Raich use marijuana that has never been bought or sold, that has never crossed state lines, and that has had no demonstrable effect on the national market for marijuana. If Congress can regulate this under the Commerce Clause, then it can regulate virtually anything--and the Federal Government is no longer one of limited and enumerated powers.

Writing for the majority, Justice Stevens lectured the plaintiffs on the democratic process. He noted that "even more important than these legal avenues is the democratic process, in which the voices of voters allied with these respondents may one day be heard in the halls of Congress." In other words, if they don't like the law, they should get Congress to change it. This is where the Seventeenth Amendment rears it's ugly head.

Depending on which number you believe, there are either 10 or 11 states which support the use of medical marijuana. That means there are 20 to 22 Senators from states where the people or the legislature shows support for the issue. If the Senate were appointed by state legislators (like they were originally), the states would not be so removed from the federal legislative process. As it is now, not only do the states have no rights that the federal government cannot overturn using their loose interpretation of the Interstate Commerce Clause, but they don't even have a voice in federal politics any more.