I'm going to install a 9kW solar grid on my roof in the next couple of months - capital cost is ~$40K USD. Without the Power Wall, I'd be selling power back to the utility at around $0.10 / kWh, and buying it back at $0.09 / kWh in the evenings. The Power Wall will also let me endure grid outages as well (you can't run your house on your solar panels directly). I'm really curious about what the end-user cost is going…
Is it really worth it if it really takes 10 years to break even (and that's the lifespan as you mentioned)? That's like giving someone a 10 year 0% interest loan. Maybe you're better off buying a bunch of Tesla stock with that money if you think they'll stick around that long.
But if you want to pay a little to be able to have reliable power from unreliable sources, then it could be "worth it". It may save you zero dollars over ten years, but during that time you're provided the service from this system.