Tried most of the abacus apps... This one looks and feels the best of them all.
It is very simple, but offers an ideal level of touch sensitivity and fills the screen making it easy to use. It has a movable index bar, to set the decimal place, and features both the Chinese and Japanese forms of abacus.
One change I would like to see is for the abacus to display differently on the iPhone than it does on the iPad. 9 rods, and the pull down menu along the side, result in the beads being a little too small for facile use on the phone. The abacus itself should take up all the screen on the phone, with 8 rods, for a bead size that is truly usable... with a separate screen for settings, or the settings moved to the Setting app proper.
Also, please add a simple reset button or touch point that racks all beads away from the index bar.