Here's the math I employed in my decision:
I found a '12 SL for $12500, and have seen others for sale in my area for similar prices. Let's assume I get a deal on a lease: zero down (after gov't incentives) and $150/mo. for 36 months. That means I'll be paying $5400 for the lease. Now, when the lease is up, if I wanted to buy the car, I'd still probably have to pay around $13-15k to purchase it from Nissan (maybe more for an SL). I bought a car that was 2.5 years old, which means that the car has probably a good 4-5 years left before it gets to 70% capacity (in my area). At that point, I will pay $5750 or so for the new battery, or less, if the tech continues to get cheaper and Nissan decides to charge less for the replacement.
Therefore, in my personal situation, I probably get an extra few years of usability out of it by buying a used one vs. leasing and then purchasing. Of course, the states' incentives vary wildly, and so you'll need to see what works for you in your situation. You might end up with a lease that is even more favorable, but eventually these subsidies will go away, and when that happens, the residual might increase.