Well, English is not my native language (Siri is STILL not available in my native language), but I don't really have a problem to use it in English anyway. The problem is that Siri is not very smart and quick to do something. The only time I use it is when I don't want to get up from bed and turn on the alarm. I just say "Hey Siri, wake me up at...". But you know what? I keep my phone in battery saving mode and there is no way to turn on the "Hey Siri" if it's in the battery saving mode. I know it sounds silly, but I think there should be something like a custom battery saving mode and give user a possibility to control what is disabled. You can turn off all those battery draining features manually, but that's not as quick as turn on the battery saving mode.
I think Siri is better in US, because of all the connected apps, but in Europe – I don't really see the pros of using Siri. And the funny thing is that Siri doesn't understand when I say "Czech republic" (my country). I was trying to pronounce it so many ways, but Siri still thinks that I say "check republic". Maybe I'm just stupid...
...but dammit, you can't even control Spotify. Ok, I know that is not the Apple's problem.