I have a 6 year old version of this guy:
Canadian Tire Link
The built in inverter died about 2 years ago, so the 120v portion no longer works.
The power pack has been used to boost an '88 Chev truck in the dead of winter, as well as my Grand Am with a completely dead battery. (Dead as in replaced the battery the next day.)
Keep in mind the limitations of the 120v power. You won't be able to run most motors; nor any heating device. But nothing beats being able to put on lights for a late night camping setup or cooking.
The only drawback to mine (and they may have corrected this) is that to charge it using AC power takes a full day; but it can be fully recharged in 90 minutes using 12 volt like a cigarette lighter. (Or as I did when I lost the chargers; a battery charger hooked up to the jumper cables.)