This time around, I got my 4S early in the adoption cycle. So I remember when Siri mostly worked.
Since then, Siri works, at best, about half the time. She's overloaded. Even when I get through processing seems more error prone, perhaps because accuracy has been sacrificed to manage capacity.
Since the initial results were pretty decent, I assume Siri will eventually work. We've seen this before; it has taken about two years for Facetime to become a useful solution.
For now I've learned to avoid Siri during the US evenings. During the mornings results are much better. I've also learned to break my requests into stages, allowing Siri to scope her language processing in smaller chunks. To create a reminder I start with 'remind me' ... then I wait ... then the reminder text ... then the time ... then I have to wait for the confirm.
Processing aside, there is obvious room for improvement. We need, we REALLY need, a way to tell Siri to give up and start over again. We need a way to tell Siri 'yes and confirm' so we can skip the confirmation dialog. I assume Apple omitted these commands because they don't market well - they expose the limitations of 2011 Siri. Just like Graffiti exposed the limitations of 1990s handwriting recognition. Time to give a bit so we can get better results from a useful tool.