I have the G6 since April 2017 when it first came out and still loving it-
1) I am light on voice-calling but heavy on texting/whatsapp, and all works fine
2) the camera is great, absolutely love the wide angle mode. Quality of the pics is not bad either.
3) The fast charging feature charges the phone in minimal time.
4) The only incident I had was boot loop caused by a defective component of the phone (so I was told), contacted LG, sent it in and repaired. Everything works great since.
Looking forward to the Oreo update supposingly coming soon.
The real big change is the LG G6 has a slightly bigger screen and the megapixel on the rear camera is different.
If I remember correctly the LG G5 has 13 megapixel on the regular camera and 8 megapixel on the wide camera and 8 megapixel on the front camera. The LG G6 has 16 Megapixel on the rear cameras and 5 megapixel on the front camera. In terms of photo quality, my wife notices that the LG G5 appears to take photos with more accurate color.
The G6 has wireless charging and the G5 does not. The G5 has an IR blaster build in.
In terms of battery life, both phones should last you more than 9 - 12 hours of use. Do note that the battery on the G5 is removable.