Not too thick in a living area. Too thick enables the carpet to move too much and it wears quicker. We have 11 mm Cloud Nine under the carpet in our temporary(*) living space over concrete probably without insulation. It's comfortable and warm but is too soft, nice in a bedroom. A living space ought to have 9 mm. This is also a polyurethane foam underlay, chopped up soft PU foam bonded into a flat sheet and faced with a plastic film. Draught proof and durable unlike the "rubber crumb" types.
(*) As in the last two years whilst we refurb...