My husband has one in his truck. Last season, him and 2 buddys were tailgating - and when they got back to the truck after the game, his truck needed a jump.
His friends were freaking out (the parkling lot was almost cleared out) and my husband kept saying "no problem". He pulled out the Halo. His friends were saying there is no way that little thing is going to jump start the truck.
Well, it WORKED! Started the truck no problem.
So, it really works.
We also like to have one around the house for when the power goes out. I can use it to charge phones and tablets.