Learning curve to using it, but it does work. At first I was going to return it, but through trial and error got it to work. It has a hard time with very low charge (dead) batteries in cold weather.
When the battery is very low, I've had success with (1) attaching the red(+)/black(-) clamps straight to the respective terminals of the battery, (2) pressing the power button until the green LED lights up, (3) then waiting 2-5 minutes with it connected to the battery. I look for a drop in power level percentage on the jumper display (5% drop or so). Once that happens, I haven't had an issue getting a battery to jump.
I haven't had any success when attaching the black (-) to the car body/engine block yet.