I picked up one of the Hyperion EOS Sentry Battery Checkers at the 2011 Arizona Electric Festival vendor area.
Once plugged in, the Sentry will display several handy sets of data for the electric flyer. The opening screen displays the number of cells, the total remaining voltage and the percent of the battery's capacity that remains. This is shown both as a number and along a graphic display along the bottom. When the mode button is pressed, the displays toggles through voltage display of the max voltage difference from the highest cell to the lowest, the voltage of the lowest cell and the voltage of the highest cell. This allows you to keep track of the cell deviations if you like to closely monitor the health of you battery cells. When pressing the cell button, the display rotates through the voltage of each of the connected cells.
I really like the percentage of capacity figure. I usually note the time flown in my battery logbook and compare it to mAh of capacity my chargers put into batteries when recharging. I'll note this and adjust my flight time accordingly. I try to avoid using more than about 75% of the battery' rated capacity on any flight. With the Sentry, I can instantly know what the capacity level is after landing.
I sometimes have checked the voltage of cells with a multimeter and almost always use a balancing charger or a "Blinky" when charging. the Sentry allows me to easily check the each individual cell's status both before or after flight. While not a balancer, it will let me know whether I need to attach the "Blinky" to balance the pack. For about $20 it's a very useful tool.