To really utilize Growl you'll need to run a few applications that will make use of it (eg. GrowliTunes for changing songs, Adium for messages - I use them for when I'm in another program so I can read them or ignore them as I work in another application without having to switch programs.)
There are also several styles so you can choose one that suits your tastes. Personally I use the Crystal theme, but if you want Growl to work I found you have to go a bit out of your way to find which theme works the best for you. Kind of like Adium, the program at default works, but shines amazingly if you take some time making it "yours".
I still remember Macnutt.