I often find Krugman a little over the top -- it seems so hard to be a pundit and maintain perceptions of objectivity -- but he has done a fair job addressing some of my doubts about the stimulus in his 9/3/2010 column. To wit:
1. We do not (yet) see the most watched-for harbingers of strain that we would expect to see if the markets regarded U.S. fiscal policy as excessive, rising interest rates and inflation. What we are seeing, in fact, are the telltale signs of deflation. Whether or not the appearance of deflation calls for more fiscal stimulus may be an open question, I think, but it certainly isn't obvious that fiscal restraint is called for.
2. The claim I've seen recently that Germany's experience shows that fiscal restraint is the better salve is laid to rest by Krugman, in my opinion. One could as easily claim that their more robust safety net, unions, and labor-management relationships has pulled them through. These features, common to some degree in other European countries, give their economy more potent automatic stabilization a priori, so that less explicit stimulus may be warranted. Oh, and their GDP has fallen further than ours in percentage terms, anyway, according to Krugman. 
[Interestingly, Krugman does not mention the fact that conditions in foreign exchange markets are extremely favorable to European exporters in his rebuttal. A recent article on the Times Business pages describes the large devaluation of the Euro relative to the U.S. and Japanese currencies, and the boon that this has been for Germans doing business in China.]
3. It's a fair argument that the second dip (as in "double dip recession") threatens because the fiscal stimulus that has already been dealt is wearing off, and more of the same is warranted. I wouldn't dare to be a strong proponent of that, but the story is internally consistent, at least with respect to the timing of events.

So OK, Professor Krugman, some good points... but more spending? Are you sure?
[For the record, I'm for the status quo. (That is, extend the tax cuts, at least for the middle class; no draconian program cuts; allow for some modest stimulus, if that's what soothes the polity; etc.) What's been accomplished by various monetary and fiscal stimulus measures has been effective. We have thwarted financial calamity. But the debt matters, and I don't think the second dip poses the same danger that sense. Financial markets seem to have accepted the stimulus up to now, but that can change in a hurry. I think that improving business confidence is the only think that will lead us the rest of the way out of the darkness.]