I've got one, it works really well. Keeps the pressure at a constant 80 psi even if I use the washing machine. TDS is 9 going into the resin (395 at tap) without the pump the TDS is 13-14.
Also without pump produces 25 liters of water per hour, but with the pump produces 35 liters per hour (300 GPD system).
Can increase pressure to 120 psi but I don't know how this would affect the membranes.
Colin