You can push anything, server notification, facebook notification, twitter replies or push mail.. as it is based on Growl. Pretty clever!

I wonder how it works out for the dev if users use it heavily for years paying only a one-time 3$...

Still on the fence as to how useful this will be, given that you either need:

a) A Growl-enabled computer running and connected to the Internet; or b) Your own server somewhere making API calls to create the push notifications.

It might work for iTunes Store developers to get cheap pushes to their customers, but other than server notifications (i.e "Server A is down, please ring so and so"), I'm not sure how useful this would be to the general public.

"your download has just finished" popping up in your iphone when you're on the go (or at the pool), or the latest tweets from friends (using Adium), or even getting notified when your friends log on to Xbox live are examples of some quite interesting general public things that can be done...