If every American saved the rebate and invested it in equities, we might be (ever so slightly) better off. Government can borrow at a low interest rate for us.We would be better off because we'd be investing in something with a higher rate of return, historically, than the yield of U.S. treasuries right now (the ten year is at about 3.59%). But that concept applies to anything with a higher rate of return, including corporate bonds and money markets (the best of which are hovering around 4.46%. If everyone was conservative and invested in money markets, and the stimulus was $150 billion, that move would create about $1.3 billion in new personal wealth after taking into account the cost of the borrowing for the government.
So if we all invest the money conservatively, we can offset the government's stupid mistake.