This is hands down the best battery pack I've used. First, the device tightly snaps into the battery compartment and gives about 20-23 hours of battery life (depending what other battery-using devices you have plugged into the GBA). The handles are big and molded to perfectly fit hands, right down to the finger rest grooves in the back. Probably the best part is the three green lights at the bottom of the device that let you know how much juice is left. These three simple lights are something I have been practically begging companies to add to the devices, and I'm glad one finally did it.
Recharging the cells is as easy as plugging the included AC adapter into the WinBattery and a wallsocket.