I had a lot of fun with it! I always love these sorts of progressive upgrades games, and I felt very slick when hitting multiple flies at once. I did have some issues with direction, I didn't realise you had to click the pc to go to the upgrade shop (though the third level being impossible without upgrades is a nice way to force you to look for it), and I didn't know you could click on the second tab of the monitor until I saw the screenshots on this page.
My biggest criticism is the zapper upgrade. It really removes most of the skill of the game and turns it just into holding the mouse button and swinging it around. The game is still fun, it provides a lot of catharsis and the squishing sounds and visuals are great still, but I had more fun with trying to hit groups of flies at once with the swatter, it felt satisfying and made me feel more skilled when I managed to hit a big group. I wish you kept the swatter the whole game.