All of our textbooks and evidences says so (I was teaching consumer theory when the vote was happening in Congress, as per usual!).  The Permanent Income Hypothesis of Milton Friedman and Life Cycle Hypothesis of Franco Modigliani, Nobel laureats for this work in 1976 and 1985, respectively (The theories have since been combined into one LC-PIH theory!), state that a permanent income change will be mostly consumed but a temporary income change will mostly be saved (or used to pay down debt, which is the same thing - lowers your "dis-saving" as we call it).

The evidence is preliminary as only $50 billion of the $100 billion has been sent to taxpayers yet, but here is the graphic of the survey from the NY Times article here:

          

The graphic says it all, 51% spend none, 49% spend all or some.  Of those spending, most goes to necessary or staple shopping!!

Furthermore, the article states that many will just use the rebate for present and future gas purchases!!