It does appear to be a little sensitive to charging amperage. I think the OE charger is 850 mA as stated above. I tried my CrackBerry charger, which has a micro-USB tip, but although it plugged in and initially indicated charging, it shut itself off. I think the BB charger was 750 mA.
I've been running mine down to 12-15% remaining juice before recharging, and then when I do charge, I leave it overnight. I've been getting about a day and a half. Worst was today (on all day, the heaviest use I've done so far), best was two full days.