Ultimately it is mental masturbation and I've personally completely lost interest because it has no functional application. And the reason why that is is because ultimately we all have to go back to our lives and live it as if we are NOT brains in a vat.
Those that believe this is an illusion, a la Inception, or a la The Matrix, inevitably end up removing themselves from the gene pool.
Even those people that contemplate "brain in vat" idea must ultimately accept the possibility of being wrong, so putting your head on a railway waiting for the train to run you over in order for you to "wake up" is ultimately a gamble that a smart and reasonable person would choose not to entertain.
So at the end of the day, no matter what your assumptions are, we all go right back to living our lives on the assumption of this reality being real, material, etc. So to the person raising this philosophical argument, I am inclined to say "cool; I am going to go right back to eating my lunch now" and chances are they will do the same.