If your accent doesn't match the country you are in, change Siri's language to your native one. As a British person living in the USA, if I don't set Siri to British, she has a terrible time understanding me, but it works fine on British mode.

Not always; if you set it to Scotland, the phone just complains that it can't understand you either in angrily incomprehensible brogue.

I think it's gotten quite a bit better over the last couple of iterations. I use it all the time and works well for most simple tasks.