Charlie Savage in the New York Times today has an extended article claiming that Barack Obama has, as the Times headline has it, had a "Shift on Executive Power." Savage talks a lot about executive orders in areas in which the president can't get Congress to do what he wants, and also makes much of the recent recess appointments despite a Senate which was notionally not in recess at all.
Marty Lederman responds sharply, arguing that there's an enormous difference between a president who uses authority that's been legitimately delegated by Congress or approved by the courts, compared with a president who attempts to undercut the law. I think that's mostly right. Presidents -- and executive branch agencies -- are certainly going to interpret the law as they see fit, and that's exactly how the system is supposed to work (as are Congressional attempts to force implementation along the lines that they prefer). Fights among the White House, Congress, bureaucrats, interest groups, and others over exactly how policy will be enacted are normal. What was unusual about George W. Bush's approach to signing statements wasn't attempts to implement laws in the ways he understood them to have been written; it was his unilateral declaration that he was just going to treat portions of laws that he himself was signing as null and void.
On the other hand, aggressive use of the legitimate powers of the presidency are, I would argue, quite a good thing. The presidency is a Constitutionally weak office that individual presidents can make very influential through careful use of Constitutional and statutory powers (such as executive orders) and through political skill. And that, as Richard Neustadt argued long ago, is one of the best hopes for what Neustadt called "viable public policy." At least it is if also combined with a vigorous Congress, courts, and other players in the system.
My own sense of these things is that the biggest problems come when a president treats others within the system -- the courts, executive branch agencies that resist the president's policies -- as less legitimate than the White House, and take actions to match that attitude. As Lederman says, it's one thing to argue in court that a law is unconstitutional; it's quite another to threaten not to carry out a court order. On the whole, I don't think that Savage makes a case that Obama has crossed that line.