The key to making Growl good is to prune what it notifies for. What do you actually care about? Me, I mostly use it as an iTunes track info display, just a little note in the bottom corner of my screen that fades in and out quietly, and reappears when I hit a hotkey. I've also used it to display error messages when debugging an Applescript. There's one guy who uses it to [give himself instant visual feedback when he breaks the build](http://szeryf.wordpress.com/2007/07/30/way-beyond-cool-autot...).
I don't have it notify me with info about every email. I don't have it tell me when someone IMs me. If an app starts spamming me with notifications about something I think is unimportant, I go into Growl's prefs and turn that app off.
I use Growl for exactly one thing: To notify me of deliveries via the Delivery Status widget. Because if I don't act on picking them up quickly, I might have to wait 1 to 2 days to get my package from the office.
Everything else? Off, because there's no material benefit for me being notified.
I don't have it notify me with info about every email. I don't have it tell me when someone IMs me. If an app starts spamming me with notifications about something I think is unimportant, I go into Growl's prefs and turn that app off.