[LG G6 Questions and Answers] frontjab:
I upgraded from a G3 to a G6 on Sprint. I haven't had it long enough to answer all your questions, but I can offer a little input.

The battery life is significantly better. Obviously, there is a lot of degradation given the G3 is several years old, but I went out and bought a new G3 battery a year ago. The G6 gets me through a whole day easy. I do use an Android Wear watch to triage notifications, so I have a less than average screen time on the phone, but I'll go running using the phone's GPS in the morning or do an hour of lifting with the screen on the whole time inputting into an exercise app, etc. The fast charge on the G6 is amazing. It is multiples faster charging than the G3. I also bought a wireless charger, which is great to set on my desk at work.

I think a case is important. The glass back is gorgeous, but I've only seen it for about 10 seconds once while I held it 6 inches over a plush carpet to install the case. I dunno. I put on a Spigen carbon case, and it does not add much bulk to the phone. I had a Spigen on my G3 for three years and it looked like it went through a war. The G3 looks pristine.

Lack of IR blaster wasn't a problem for me because I just took my old G3 and put it in the TV room to use as a remote. The only issue I see with the non-removable battery is getting a few years down the road when its capacity is less.

I don't use YouTube much at all. I did just now pop it open and fired up a video, and can confirm it did not stretch to full screen. I thought I read somewhere that Google was going to work on this on the software end as they're expecting more 16:9 phones to come along. I have no idea what the S8 does.

I haven't tested out the camera much. Honestly, I was amazed at the high quality of the G3 camera and would be happy if the G6 was as good. I take videos of our kid's wrestling matches and share some of the videos with other parents when they can't be there to watch their kids. I have gotten a lot of remarks about how fantastic my phone's videos are (the G3, that is) from people that have iPhone 6's and 7's. Still photos were awesome from the G3, and I should drag my butt out and try out the G6's new wide view.

I can't think of any other downsides. I do like the form factor of the G6 a lot more than the G3. I didn't mind the G3 that much, but the G6 is a lot narrower. It feels much more comfortable in the hand than the G3. It's taller, but that doesn't seem to be much of an issue for me. Fingerprint scanner on the G6 is great. Unlocks the phone instantly. It was a big adjustment to get used to volume buttons on the back of the G3, and now it's a big adjustment to get used to the "normal" side buttons of the G6. The G6 doesn't seem crazy faster than the G3 and the screen resolution is the same. The G6 does look a little sharper and clearer. My Sprint G6 is supposed to have this awesome new HPUE that increases speeds inside buildings, but I haven't noticed much of a difference in LTE speeds. Maybe that will come.

My G3 was stuck on Marshmallow, and the G6 comes with Nougat and Google Assistant out of the box. I haven't gotten into any good conversations with Assistant yet.