I hear ya, man! I was so ****** off with Siri's increasing unavailability after the 5s launch, that I emailed Tim Cook about the issue. Apple contacted me two days later, and have been very professionally working with me to resolve the issue.
One thing I would suggest, that seemed to help me, is to reset network settings: Settings/General/Reset/Reset Network Settings.
These past few weeks, Siri has been working very well for me with virtually no unavailability, although it is sometimes slower to respond to my request than usual. I've made a recommendation to Apple on a workaround for the slow response issue. The more direct feedback Apple gets, the higher priority and likelihood your issue is addressed. So, please give feedback to: