I use Siri regularly, but just for setting reminders, alarms, and timers, simple things that doesn't make me sound like a dick talking to my phone. For instance, every morning a 2 min timer for brewing my tea, or setting a wakeup timer for the morning. These are all pretty reliable. You learn to specify "pm" or "am" when giving a time, or it might get it wrong. I feel self-conscious talking to my phone, so don't do it in public. Occasionally I have asked it a trivia question and got a useful answer, like the population of somewhere, but generally the longer a question or more exotic words used, the more likely it is not to get the whole question. Placenames and the like in particular. I can't imagine relying on it to play things from my playlist, or movie titles. I learned a little while back you can teach it pronunciations, which is pretty cool for getting names pronounced correctly. For instance, my wife's surname of Austrian origin, which hardly anyone in the human world gets right.
Siri has gotten better over time, whether by learning my particular accent (Danish background living in Australia; even people sometimes aren't sure what I said with certain sounds), or by general improvements. I expect both. But if it learns your particular voice and accent, then considering it is all done in the cloud, they have voice profiles of all users. Must be one hell of a database. From what I hear from overseas videos, like the US, the Australian voice is actually one of the better ones (I use the female voice).