Siri understands my voice correctly most of the time, although it falters with place names, particularly those it has never heard before. Street names or place names, for example. But that's kind of to be expected.
What I find I use Siri most often for is to set an alarm. It's SO much easier to say "wake me up at 7:30" than to scroll over to the first app screen, open the Clock app, navigate to the Alarms page, hit Edit, scroll the numbers to 7:30, and hit Save. Especially late at night when I might already be lying in bed.
Otherwise, I keep my requests pretty simple, understanding that "she" doesn't really understand anything, and is basically pulling out the most important keywords to base its response from. "Siri, would you please be a dear and tell me what alarms I have set?" and "Alarms" both show the same results. (But, thanks to the magic of Apple engineers with apparently too much time on their hands, the first one gave me a more interesting response first...)