I primarily use Siri in the car. However, there are certain things I frequently do that Siri is just faster at, especially now with "Hey Siri."
For example, adding items I think of randomly to my reminder list, which is then farmed by OmniFocus 2. When at my desk, it's even smoother as I frequently have my iPad near me, plugged in, with Hey Siri enabled. If I'm in my car it's similar, "Hey Siri...<wait for bell>remember to write a message to my mother for mother's day" or perhaps adding a grocery item to the grocery list or whatever. Even when not in the car, it's way faster to do this via siri as this is the kind of thing that, even if the dictation gets it a little off, it doesn't matter. Only I will be reading it later and it will still make sense to me. That said, dictation is spot on 90% of the time for me. Probably more.
I also do many other things with Siri when driving, such as ask for restaurant locations nearby by category "Where's a good taco place around here?" etc.
Siri doesn't replace basic direct operation of the device most of the time, it's something that works very well for me in certain situations and I'm glad for it. It really does feel like an assistant as it is performing operations alongside me doing something else in most cases, like adding reminders. I'm usually busy and just want to get that idea/thought down ASAP.