I have a the GB40. Works as intended. I have used it numerous times the past 4 months of winter. It has a override mode so you can start a vehicle that has no battery or a vehicle with defective battery.
Boost pack and jump and carry made by solar are the best I believe. I work at a towing company and all the other Jump packs don't last. The company I work at hire people that can destroy a sledge hammer but the boost packs just hold up longer