NO!
It's the most pointless and irritating application in existence! You all know those stupid pop-up balloons that come out of Windows tray to tell you stuff like "you plugged in a usb-stick" and you go like "oh no way I didn't notice?!" in very sarcastic manner. Growl is that, just worse. Click "next" in iTunes and Growl tells you that "hey dude j00 play anuther track" and you are like "right, I meant to u freaking brick". First thing I do when installing Windows is disable the tray pop-ups from registry altogether and first thing I don't do in Mac is install Growl.