The jump packs I have used are ,as mentioned, a helper not a stand alone unit. I work at the Tire and Lube at Wally world, so I get to use the jump-pack at work a lot. It works about 75% of the time. I have found hooking up the pack, try it, if no dice then wait 5 minutes and try again helps. Personally I dislike jump packs, they don't pack the wallop that my little ranger does. experience has taught me jumper cables are the way to go for dependability. I have only seen jumper cables fail once (short in the line of a set older than me) but I had mine so no worries.
Of course you could just drive a manual transmission and roll start it. (guilty as charged, battery died a couple days before payday, so i parked facing downhill until i got paid )