With the government incentives (both state and federal) our 10kW system paid for itself in 3 1/2 years. I live in Washington State, so not the sunniest state around.
This is also largely because we installed it ourselves (total cost ~$15k) instead of having someone else install it ($40-45k). It took my father-in-law and me ~4 days to install. I have been very happy I did it. Now my power bill is ~$200-250/yr for a 6,000 sqft house.