Has anyone used one of these?

Some do but have heard of plenty saying no because of safety / in case of emergency evacuation etc x

Most airlines don't allow it, I have the plane pal instead, I have been allowed to use this one but once we take off and seat belts lights go off, also you are not allowed to put it on a middle or aisle seat only on the window one the only exception is the middle row where you can put it in the middle seat this is because of safety measures.
And it needs to be taken off prior landing.

I heard of an inflatable cube which goes in the leg/foot well area same height as seat so as to make a bed, might be a good alternative