This is the closest I could find to a study about it:
http://pdj.sagepub.com/cgi/content/abstract/6/3/201The socio-economic impacts of microcredits on women's welfare were studied in northeastern South Africa using a qualitative semi-structured interviewing method. The study focuses particularly on the benefits, disadvantages and challenges obtained from The Small Enterprise Foundation (SEF) microcredit village programmes (Micro Credit Programme (MCP) and the T'shomisano Credit Programme (TCP)). Impacts of microcredits and a comparison of two programmes were analysed on a household level. The results revealed that a number of the poorest women have been released from deepest poverty through the opportunities provided by SEF. The MCP clients, who tend to have a better starting point, seemed to be more educated, able to diversify and improve their businesses as well as being more capable of protecting themselves against vulnerability than the poorer TCP clients. Many repayment problems were caused by group heterogeneity. Particularly in the MCP, the incentive to free-ride seemed to be a problem phenomenon. These drawbacks demonstrate that there are still some unresolved disadvantages in the programmes.
Sadly the full paper is gated and requires either a subscription or a fee to access it for a day. Without the paper, there's no way to know what could be responsible for the free-ride problem. From what I've read and heard, though, in group loans there appears to be a significant pressure to keep up with payments from the rest of the group. (as they'd otherwise be liable for it) It's hard to tell for individual loans, because we don't really know if a borrower actually is making repayments, or if he (or she) has defaulted and the MFI is making the repayment from its own money. They have an incentive to do so, as the repayment rate is important for them to attract more money (not just through Kiva) at better conditions. If they can avoid a loan defaulting by spending $100, that can be a smart move.
edit:
Interestingly, I can't find anything about the impact on the communal level. Let's say people in a village of a few hundred people start to have access to microloans. Does this boost the welfare of everyone in the village? (Fisher receives a loan, sells more fish in the city, spends the money in his local town?) Looks like much research remains to be done in this area.