Clark Neilly, Institute for Justice: "Today's outcome is a tremendous victory for liberty, and we are pleased that it hinges on Justice Thomas's compelling account of the history and purpose of the 14th Amendment, including the central role of the Privileges or Immunities Clause."
Ilya Shapiro, Cato Institute: "Thomas's clarion call for a libertarian originalism provides a step on which to build in future."
Josh Blackman, Harlan Institute: "For the first time in the history of the Supreme Court, a Justice found that an essential liberty is protected by the Privileges or Immunities Clause."
Timothy Sandefur, Pacific Legal Foundation: "While some parts of the decision are disappointing, the bottom line is that the state of Illinois may not legally disarm law-abiding citizens. They have a constitutional right to defend themselves from criminals who never have obeyed anti-gun laws themselves."
Glenn Reynolds, Instapundit.com: "I'd like to note that a lot of 'respectable' commentators were, just a few years ago, calling the individual-rights theory of the Second Amendment absurd, ridiculous, and something that only (probably paid) shills for the NRA would espouse."
David Rittgers at National Review: "The McDonald decision is a harbinger for the end of gun prohibition as an idea. The simple, undeniable truth is that gun control does not work."
Randy Barnett at SCOTUSblog: "Neither Justice Alito for the plurality, nor Justices Stevens or Breyer in dissent, even attempted to impeach Justice Thomas' analysis, which now stands uncontradicted in the Supreme Court Reports."
My preliminary take on Justice Stevens' dissent is here.