I've used Kiva for a couple of years, rolling over the repayments (which have been pretty consistent) into new loans. I honestly can't say how much of a difference it makes, but it's fairly painless and I suppose there's a chance that it's really helping someone out who can use the money more than I can. Everything I've seen about Kiva as an organization says that they're pretty above-board, that they don't take an inordinate amount of money in overhead, etc. Each time you make a loan you're prompted to bump it up a buck or so to help fund Kiva itself, which I believe is mostly how they keep themselves going.
I certainly wouldn't look at Kiva as any sort of reliable investment if you're primarily concerned with making your money grow, though. More like an easy and somewhat fun means of charitable giving.
Out of curiosity, just looked up my statistics. I've put $100 in the system, which has resulted in $325 of loans to people over time. Out of that, I've lost $11 from people defaulting. This is over about a 2 year period.