Orin Kerr makes a great point about presidential signing statements here. It's a grave problem that the president has decided that his Article II powers exempt him from having to obey every single provision of every bill signed into law, especially provisions he thinks are unconstitutional. But anotherand perhaps biggerproblem is that Bush sometimes doesn't explain what, exactly, he disagrees with. If he did, then perhaps Congress could respond appropriatelyperhaps by passing other laws to constrain the president if need be, and there might still be at least some semblance of checks and balances.
Instead, Bush just says that he's free to disagree with and disobey parts of the law, but declines to say which parts. To make the contrast clear, as a commenter at Orin Kerr's site points out, Bill Clinton occasionally used signing statements to disagree with parts of bills he thought unconstitutional (never to the extent Bush has, though), but he specified exactly what he was doing and why he disagreed. The same isn't true of Bush; basically, we have no idea what laws he thinks he can violate.