I just got mine yesterday afternoon. I'm coming from a 5s. So far it's awesome. It's everything I was hoping it would be. I like the large screen and I got plenty of storage now (128 as opposed to 16). Anyway, overall it's the greatest thing ever but I do have some comments and observations.
First, it's big. Taking it in and out of my pocket is nothing like it used to be with the 5s. Not saying that's a bad thing and this is what I wanted. But thinking about what it will be like and then actually handling this phone are two different things. And it's got some weight to it.
I got the 3M privacy guard for it. I recommend not using that. It does the job but it's a little ugly and you tend to rock the phone side to side more since it's so big. So that privacy keeps blacking out the screen as you are handling the phone. Also a stylus doesn't work with it. I never use a stylus but if I did then I couldn't with the privacy guard on.
I also got the Apple silicone case. It's not bad but kinda hard to put on and also difficult to take off if you need to do that. Also if you prop it up against a wall or something then the slick surface of the silicone case makes it slide and eventually fall flat. So not happy about that either. I think I need a different case.
3D Touch is good. I keep hearing people say it's, "useful". And yeah, it is "useful". I like the little clicking effect it has as it pops and peaks and all that. Pretty nice. Not a "must have" but good. I think this will be something we will all grow into over time.
Not sure what's causing this but here's what happens: iPhone is locked. I put in my finger on the TouchID. It logs in. Then there is some weird lag that happens before the info at the top of the screen shows up. Also can't tap on anything. Then about 2 seconds later it all starts working. Not sure if that's a byproduct of me upgrading instead of clean setup. Or some bug in 9.0.1. For now I'm just chalking it up to a software bug.
Performance is great. Then again, the 5s was nothing to sneeze at and has held up well for the last 2 years. So honestly I'm not blown away. But it's great performance as I expected. Just not earth shattering like all the popular tech blogs might lead you to believe.
Also I was a little disappointed that iPhone 6s Plus users don't get Split View. I guess that must be an iPad-only thing. Oh well. Was kinda looking forward to trying that out. I picture in picture is only for iPad users as well. Although I haven't fully looked into all that yet.
Anyway, those are my thoughts for my first 24 hours of use point of view. Besides the stuff I wrote above,.. I can't out this phone down. I love it. It's so great. Best smartphone in the world imo.