I hated Siri's lag and quiet fails for a long time. But since iOS 6, it has been a night-and-day improvement. I'm really starting to like it, and the biggest critic, my wife, is starting to use it also.
I no longer get the long waiting times and quiet fails. I get much faster response. I get much better results. IME, success went from somewhere in the 50% to about 90+%.
Still frustrating that it doesn't understand "Wreck it Ralph" even in the context of a movie search. Instead it wants to search for "Rekkit Ralph".
Current complaint: I want Siri to repeat driving instructions on demand.