We got solar in 2008 first and foremost to reduce our carbon footprint, but the fact that it's going to be cheaper than the grid over the long term is a nice bonus.
When we got our system, SolarCity estimated that our break-even point vs. the grid would be about 12 years. I haven't been keeping detailed enough records to know if we're on track for that, but assuming that's about right, it's nice to know we're half way to truly free electricity.
On average, over the course of a year, we have been netting out at close to $0/year for electricity. However, our net energy consumption from the grid has been around 1 mW/year. Because of time-of-use pricing, we get paid more for our excess generation during the day and on peak than we have to pay for energy at night and off peak, which is why our net cost is around $0 even though we're still a net consumer from the grid.
At the time we had our system installed, SolarCity was just starting to offer solar leases, but we opted to pay for our system outright. It was a while ago, so I can't remember all the details of exactly why we made that decision. If I were doing it again today I'd certainly consider a zero-down lease or loan.
Our lifetime output as of November 2014 is 43.3 mW.