I would like to share my experience as I'm just a beginner so when something works I'm happy to share!
I ordered a Mintyboost at Adafruit and first thing I want to say it's not working at all with 2 AA battery on a iphone 3gs.
I tried a couple of battery before finding one is working great !
It's a battery I have taken from a laptop who died, a Li-ion 4.2v.
AND IT WORKS!
I'm so happy.
I will give more detail on the charging time and the amount of charging when the battery will run out.
And don't forget to solder the middle usb center.