Quick & smooth
Does what you want it to do, quickly and smoothly without getting in the way. This is perfect as a menubar item (I wish there was a 4-finger gesture or something to trigger it), so you can easily and quickly access it and then put it away just as quickly.
As someone else mentioned a little x to clear input fields instead of hitting "delete delete delete" would be a nice touch, and a nice subtle way to display (as an option) previous calculations (a tape) would be a nice as well.
Definitely worthwhile at less than the cost of a fancy coffee drink at Star*ucks.
Todd G about Currencies