A few examples. I find that adding new calendar events with Siri is much faster than doing it manually.
http://spiceware.org/gallery/siri
and it has a fun sense of humor. I was showing it to somebody the other day and asked "What's the weather going to be tomorrow?" He started talking when I did and Siri misunderstood our overlapped speech to be "what's <some company> going to be tomorrow?" It brought up the current stock price and say "now Darrell, even I can't tell what the future will bring".
I tried the popular "Beam me up" and it asked "WiFi or 3G?"
other screenshots of fun responses
http://shitthatsirisays.tumblr.com/