Yeah, that's true, i'll use it as an emergency charger. A lot of my friends are students around the country (the Netherlands) and they don't have iPhones (too expensive for them), so they don't have a charger. I now can forget about my charger, because i'll always have a fresh battery charge waiting for me in the car. (Now i only need to remember to recharge the 3GJuice ;))
Just got my 2 3GJuices, a little late, but better late than never. I let them charge for a while (till the light went off) and then plugged one in, and voila, it works perfectly :) The only thing i'm a bit worried about is how it connects to my phone, it doesn't look that sturdy, so i keep in on the table or in the car-holder while it charges.
Great job, i threw one in the glove compartment (btw, this would've saved my life last weekend, when i had a long weekend away from home and i forgot my charger...).