This circuit has a very high input impedance, at least at low frequencies. Due to the input capacitance of the MOSFET, the impedance falls with increasing frequency. For example a 2n7000 has a maximum input capacitance of 50pF, at 1 KHz this is 3.18M ohms, at 20Khz ( top of audio range ) it would be as low as 159K ohms, this is the worst case scenario and there are probably RF MOSFETs with much lower input capacitances and therefore more suited to this circuit.
