A nice way of testing that PubSubHubBub really works (and how fast it is) is to subscribe to Google Reader Shared Items feed. Google Reader exposes your shared items as a PuSH enabled Atom feed. See this blog post for more details. I use my shared list for testing, and it is available at: http://www.google.com/reader/public/atom/user%2F05268996354213702508%2Fstate%2Fcom.google%2Fbroadcast
Every time you share an item via Google Reader, it sends a notification to the Hub, and the Hub in turn notifies the subscribers. And in my experience, this tends to happen very fast, in few milliseconds.
Although PubSubHubBub brings the promise of the real time Web closer to reality it is a shift from the way feeds are consumed. In vanilla Atom or RSS, all a client has to do is to poll feeds at regular intervals. This may not be very efficient, but it is quite simple to implement and only requires that the client has access to the Web. PuSH is a lot different in the sense that the client that consumes feeds is now a server (more precisely a Web Service) which is accessible from the rest of the Internet. This is a bit more difficult to implement, and has some other serious implications which I have not addressed in my exercise, such as authentication and authorization.