Although I suspect most people still use the original female Siri voice, you've had the option for a while now of changing the gender or the accent. And now the latest update to iOS 7.1 offers more natural-sounding UK and Australian English voices in both genders, so if you haven't played around with Siri's voice, this might be a fun time to try it.
To change Siri's accent or gender go to Settings>General>Siri.
Tap Language and choose the version of English you prefer to give Siri an accent.
Tap Voice Gender and select Male or Female to choose whether you prefer Siri to have a male or a female voice.