Works fine and it seems fast on a fast machine. Got an option to use opposite colors, i haven't seen this feature in some other tools. The 4 star is mainly because this program can do it's job without errors and you can get all basic help.
No help tips for basic file selecting (use Ctrl or Shift + left click). It'll save the file as .bmp, however, you may choose gif and jpg formats from File menu. Unnamed.bmp + a second extension isn't a good idea.