My experience with them falls into the category of "your mileage may vary..."
I bought a larger one that plugged in to a wall outlet to recharge. Tried it once and it didn't provide enough power to get the car started. Tried it again more recently with the same result. Maybe defective. Who knows. Tossed it in the trash.
In the latter case, it was my neighbors Toyota Sienna minivan that wouldn't start because of a dead battery. So, onto plan B. Got out my heavy duty jumper cables, connected the Sienna to his adjacent Volvo XC90. Same basic result. WTH? Finally I told my neighbor to get in his Volvo, and instead of keeping the car at idle, rev the engine to like 2,500 RPM (a bit deafening in the garage might I add!). Well, THAT did the trick. Guess we needed more than just a "battery jump" but the Volvo's alternator at max output in order to provide enough cranking power for the dead Toyota.
Neighbor gets to the Toyota dealer, and they said, "yeah, we noted your battery was weak at the last service..." and they also noted that the battery was a week out of warranty.
You don't want to know the rest of the story. This is a family friendly forum.