I don't have much to say on this, but here's John Cochrane:
In sum, there is a plausible diagnosis and a logically consistent argument under which fiscal stimulus could help: We are experiencing a strong portfolio, precautionary, and technical demand for government debt, along with a credit crunch. People want to hold less private debt and they want to save, and they want to hold Treasuries, money, or government-guaranteed debt. However, this demand can be satisfied in far greater quantity, much more quickly, much more reversibly, and without the danger of a fiscal collapse and inflation down the road, if the Fed and Treasury were simply to expand their operations of issuing treasury debt and money in exchange for high-quality private debt and especially new securitized debt.
And Scott Sumner:
We can get rid of interest on bank reserves (and consider a penalty rate), set an explicit nominal target, and engage in quite substantial quantitative easing using indexed bonds (and perhaps a few foreign government bonds) without incurring much risk at all. And even if we have to eventually move more heavily into assets more exposed to U.S. inflation risk (long term T-bonds) I don’t see how those risks are any worse that what we are now doing at the Fed. Isn’t a risky policy that has a good chance to boost AD superior to a risky policy that has little chance of achieving that goal?