They are similar in the sense that presidents use them to selectively identify which parts of a law they oppose. But since they do not have the force of law – they are, technically, merely advisory – they are not unconstitutional. Even Harvard Law Professor Larry Tribe – no fan of Bush’s – acknowledged that Bush’s use of signing statements did not break any laws. He wrote that the statements are “informative and constitutionally unobjectionable.” What Tribe and others object to is that presidents use them instead of vetoing bills. (Read more at the American Presidency Project: Presidential Signing Statements http://www.presidency.ucsb.edu/signingstatements.php#q5#ixzz1hgE4sig1)
Of course, that noted professor of constitutional law Obama has shown no reluctance to use them.

These differ from the line item veto. In its brief existence, the line item veto was actually incorporated into the lawmaking process by an act of Congress and therefore it did have the force of law, until the Supreme Court in the Clinton v. New York City case ruled the line item veto unconstitutional.