I'm only using a single Tesla Powerwall for my battery backup.  It wouldn't last long off the grid as the sole energy storage device for my operation.  I could go off grid during the day when the sun is shining though if I wanted.  That's a nice usage for it.  Also, any power outages won't immediately effect anything I'm doing.  The real benefit for mining though I would say is that I can store excess solar generated energy from sunrise to noon, and then once the sun goes down it uses that stored energy before going to the grid until 7pm when electricity on my plan becomes dirt cheap again.

The Tesla Powerwall is awesome.  I've had no problems with it whatsoever, and it provides awesome graphics and statistics about your power usage.  I would say that interface is almost more valuable than the battery itself.

The system software doesn't give an expected life for the batteries, but I believe Tesla stated it goes based off of how much usage you get from the battery.  I have mine set to only use about 40% of the battery on a regular basis as that gives me the desired effect, but if I recall from memory the batteries would last 10 years using 100% daily, so I would expect mine will probably last in the 15-20 year time frame.  By that time it is possible it would have paid for itself if electricity continues to rise, but I think being able to go off grid and continue to allow your home to function during power outages is the likely main benefit.