Signing statements are used by Presidents to circumvent Congress. The idea that this is an excessive use of Presidential power is certainly valid but for Republicans that depends more on who is President, rather than a constitutional issue.
Bush used signing statements to exempt U.S. contractors in Iraq from U.S. laws, to stop Congressional attempts to fight genocide and to the legal obligation to hire qualified people for FEMA. "You're doing a heck of a job Brownie" will live in infamy.
In a House Appropriations bill the House Republicans have barred the President from using funds doe 'signing statements'. It will in all likelihood, be stripped out of the Senate version. Just another example of IOKIYAR (It's okay if you are a Republican).