It was forcibly installed without your consent, but you did have knowledge of it. On a side note, what didn't you want about it?
It's like I said, I repeatedly declined the install until one day there was no more opting out allowed. Only an OK button and no X or "No thanks" or even "Later" option. Just "OK". When the message covers your screen and you can do nothing else on your phone until that message is cleared, in my world, that's called not having any option. As for whether or not I had knowledge of it, not sure how to answer that... seems obvious I did.
As for what I didn't want? I had a 2 week old phone that absolutely sang. Everything I wanted was at my finger tips in lightning speed. In an instant, my wi-fi connections went from immediate to 30 seconds or more and sometimes requiring multiple on/off cycles to convince it to connect. This is not to different wi-fi hotspots... this is to my home wi-fi, configured properly and running perfectly for all other phones, iPads and laptops in the house. Additionally, and like many have complained here, the internal storage, file and SD card management is beyond unworkable. My issue is a bit different from many others who complain to have a new /0/ directory emulating folders. I too have that same /0/ but my folder contents are not emulated or duplicated. The folders themselves are but not the files... for instance, in Gallery, I have an Album called "Kids". If I take a picture of my kids it will initially go to an album called "SD Card" by default. I go into that folder, select my new photo, select the three dot menu, select "Move to" and choose the "Kids" album. Once complete, it brings me to my albums list again and I see two "Kids" albums... one with all the previous kids photos and one with only the new photo. If I try to move it again, I then have two "Kids" albums in my list and no matter how often I try, I cannot get them all together into just one.
I've tried going and moving it physically using ES File Explorer and when I cut it out of the one folder, by the time I navigate to the second (original) kids folder, I can no longer paste the photo and it is lost forever.
I also tried copying files rather than cutting and when I go back to delete the ones from the folder I don't want, it deletes all of them.
And all of the above can be repeated for my eMails as well except in gMail it's "Tags" not folders and eMails will randomly appear in my Inbox or, most often it will say "You're all done here, have a nice day!" and yet I know I received mail because I can see it on my laptop. A day or two later, it might pop up under some random tag I never asked for, or, better still, I will click on "All Mail" and get swamped with dozens of unread eMails. I've seen many forums with people complaining about gMail tags so perhaps that's not related to Marshmallow. But again, I had everything working perfectly for two weeks before all this mess started. If gMail issued a release at the same time as the new Android system, then I guess they're both messed up.
Additionally, I cannot predict whether or not my camera will be trying to write to internal storage even though I've set it to use the SD card, out of the blue it will ditch my recorded item due to lack of space caused by all these apps that have inexplicably been permanently moved off my SD card and have used up all the real estate on my internal storage.
If I had the wherewithal to have taken a screen shot of the "no option, we're ramming this marshmallow down your throat" I highly doubt I would have been able to locate it to post it here.