I don't think the answer is clear yet, but people are finally applying a little scientific method to the problem (especially properly randomised trials).
A number of recent studies are referenced on this review site (my emphasis).
One review study by the grameen foundation in 2010 (seemingly a fair summary and updating the summary quoted in another answer) reports:
Two of these studies (“The miracle of microfinance? Evidence from a randomized evaluation,”  by Abhijit Banerjee et al. and “Savings Constraints and Microenterprise Development: Evidence from a Field Experiment in Kenya,” 
by Pascaline Dupas and Jonathan Robinson), when examined closely, report evidence of a number of positive impacts of microfinance on the lives of poor clients. Banerjee et al., find that the introduction of microcredit
in Hyderabad, India, supports household borrowing and investment and supports the creation and expansion of small businesses. Dupas and Robinson study the effect of the introduction of savings accounts on business investment in Kenya, and find that formal savings accounts increase business investment and expenditure for women. The third study (“Expanding Microenterprise Credit Access: Using Randomized Supply Decisions
to Estimate the Impacts in Manila” [2010b] by Dean Karlan and Jonathan Zinman) finds that the expansion of microlending to a new population in Manila, Philippines, leads to an increase in business profits for male borrowers only but has no overall effects on income or poverty. Banerjee et al. and Karlan and Zinman both
test for, but do not find, evidence of social impacts of microcredit (such as women’s empowerment, increases in children’s school enrollment, or improvements in overall health and well-being). Collectively, these
three studies suggest that over relatively short time periods, microfinance had positive impacts on business investment and outcomes but did not have impacts (positive or negative) on broader measures of poverty and social well-being.
So thats a fairly muted positive for micro finance. It isn't a miracle, but it seems to be better than nothing.
The topic is a lot bigger than can easily be summarised. It is worth reading Banerjee and Duflo's book on the subject, Poor Economics to get a broader overview.