I just added links to both the Microfinance Open Book blog by David Roodman and the Innovations for Poverty Action blog (look to the right for my list of suggested blogs). Others more expert than I have weighed in on the debate of the true impact of microfinance on the poor (which I believe is a healthy one). And these two blogs are a great way to understand the debate. I'd start here.
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,