When I worked at Nissan we used them all the time for cars with flat batteries from electrical testing. The key things are current, it does take hundreds of amp to start a car, and lead size. This one seems a bit cheap but the specs look OK. Since it has a battery and was probably made in China then how it was maintained during shipping is critical. This may explain the varying reviews.
If you leave your car with a flat battery for long periods it degrades the cells so generally you need a new battery but this will at least start the car so you can drive to Halfords and buy one.