Being a ham radio guy into portable operations, I keep a motorcycle battery sized sealed battery around and well charged. It's not always in the car, but if I'm headed to the sticks, I sure want it with me. It was about $80 and most of the time I just use a $25 2amp charger for it.

The sealed lead-acid batteries in almost all of those things need pretty tender loving care. Charge them with more than 14.4 volts and you cook them. Let them drain too low (~10.5v under load / 12v idle) and they're shot. I'm pretty careful with my batteries but I still fry about one a year now. Those cheap wall-wart chargers on most of those jumper packs are asking for trouble. You're better off with one of the packs that you can plug into the cig lighter to charge, since that's a nice regulated voltage source. (Any spikes when starting aren't too horrible for batteries, but a long-term over-voltage is fatal).