I had a Nexus 6 until I dropped it and wrecked it, and I loved using Google Now on it. Was years ahead of Siri.
Back to using Siri on my iPhone, and it's pretty bad. I have "Cox" in my contacts (my cable provider). The other day I said "Call Cox" and it started to call 911 (it interpreted me as saying "Call Cops"). I do not have "Cops" programmed as a contact. I guess that is actually "smart" in that it figures out what you mean by cops (not that I said cops, which has a bilabial stop in the /p/ as opposed to Cox which has a velar stop in the /k/ sound). It also still thinks my nearest emergency room is a veterinary hospital or a museum of an old psychiatric hospital, which is a problem caused by it using Apple Maps--an issue I have been reporting for years to Apple through the Maps app. I've never actually needed it to find me an emergency room, but I've tested out if it would know in an emergency. It varies based on whether you say "emergency room" or emergency department" and both times it gives bad results.
Sometimes it's just really stupid and like Mossberg said it gets all the info right and still makes a mistake that is local to the device. For example, I'll say "Call Joe Smith" and it transcribes it right, but says there is no one by that name in my contacts even though there is. Other times it will say, "Did you mean call Joe Smith?" even though up above it shows that's exactly what I had just said.
As far as transcribing correctly, the Nexus 6 was also way faster and way more accurate.
A problem that Apple has is that it doesn't know or care that it has problems. I've been telling Apple that their software has been in decline since whenever it was that Lion came out. But they were in an ascendency so great, I don't think they could see that their quality was declining. At least in the 1990s, they knew that they had problems and they were trying to find solutions. Buying NeXT and bringing on Steve Jobs was one of the smartest things they did. But even before that they were acknowledging issues with Mac OS and had various plans to try to come up with a next generation OS. Now they're too rich to care.