I like it a lot. Very basic arithmetic functions only, but enough for what I need it for (some quick additions and subtractions). Big plus in my book: copy & paste works both for entering values and getting the result (many other add-ons only allow one of the two), and it understands grouping of 3 numbers by comma (didn't find any other add-on that does that).

Works with what it knows, but it was pretty annoying when I realized that it can't multiply negative numbers.