After 2 years of using Android, going from love to hate, I returned to iPhone with the 4S. I hadn't even heard of Siri til I was leaving the store with the 4S and noticed Siri mentioned on a poster.
Siri is useful in a very limited number of circumstances. I routinely use Siri to set an alarm. S/he seems to be good at understanding stock market enquiries too. But the natural language parsing can be very random at times. For example, try "set a countdown for 10 minutes" -- sometimes you'll get "I don't understand", sometimes you'll get an alarm clock set for 10 mins from now, and sometimes you'll get what you want which is a timer counting down from 10 minutes. Try "set a timer for 10 minutes" and you'll get the same range of mis-understanding.
I'm fine with Siri being how it is at the moment. I know it will get better and more useful, especially when it can work with maps / businesses outside the US. But it is still definitely a beta product that is usually slower than performing the task yourself.
Siri in a year or two should be great. I'm looking forward to it.