Is it immediate? Sure, there is satisfaction that you are using 'free' electricity. But it does have an upfront cost. I calculated that it would take over 11 years to recoup the investment based on our current usage. Given we already get cheap night-time rates to charge the car and run appliances, it is hard to justify.
Like many UK houses, we have gas central heating too. I guess if we had a battery too (more investment) then we could switch to using electric oil-filled radiators, though they would not heat the whole house. And we could install a hot water tank.
I guess for new builds there is a real opportunity, but for an existing household I'm struggling to see how it works - and I want it to!