While the functionality is good (it's nice to have a visual warning when your PowerBook power supply is disconnected), this app is definitely NOT the one to use.
Problem: the application prevents the PowerBook from sleeping (Mac OS X 10.4.6 at least). I did extensive troubleshooting and finally attributed the problem to this app.
Second problem: ChargerChecker prevents the disk drive from spinning down. In addition to draining the battery more quickly (my life dropped 15-20 minutes when running this app), it made estimates of battery life unreliable.
Third problem: actually an issue rather than functional problem. This application phones home (Little Snitch revealed this).