For jeans, the back pockets were fine for me and the 6s Plus, but flex-gate was a thing, so I would try to avoid using them an accidentally sitting on my phone. Front pockets were tight, but it really depended on the brand of pants I was wearing at the time.
Dress pants for work were actually good to fit the phone in (all mine have deep front pockets, but once in, the large phone would stand out like crazy, like I was hiding a notebook in my pants. I never use the back pockets on my dress pants.
The real problem for me was with coats.
My favorite coat has breast pockets with buttoned flaps. I use the left breast pocket for my phone, and it was perfect size for my original iPhone and the 4s. The 5s and SE1 fits fine as well, although buttoning close the pocket (prevented the phone from falling out when leaning over) started to get a little tight.
The 6s Plus stuck out of the pocket, and I couldn't close it, not even close.
Not only did I have to be careful when leaning over, but the camera was often pointing away from my chest, since this was the natural way for me to put it back in my pocket after using it. It was not unusual for people to ask me if I was recording a video.
I went shopping for a new coat, and finding a breast pocket that was deep enough to hide the iPhone 6s Plus was the main criteria when shopping around, besides it being comfortable.
Shopping for clothing that specifically fit my giant iPhone wasn't the only downside of the 6s Plus' size, if this was the only thing I didn't like about it, I probably would have adjusted.
I have a lot of cars, and many of them are older cars that don't have a place to hold a large phone. I would often have to place it on the seat, which sometimes the phone would fly off when going around a turn fast.
I also had issues using the 6s Plus one handed, especially when trying to take photos.
This wasn't an issue with the original, 4s, 5s, and SE1.