I've used both the Gsam (aka Badass) battery monitor and "Battery HD." I prefer the latter on my KFHD and purchased the paid version mainly to reward the developer for an excellent app. The UI on the KFHD is excelllent; the calibration is extensive; and the estimated time available for various activities is largely accurate.
One of the features I like most is the fact that the app can be configured to overlay the Carousel UI when the battery is being charged. Since the KFHD lacks the illuminated power button indicating the state of charge, this provides a relatively good workaround for determining the exact state of the charge. And when the battery is full, the app announces the fact.