After having a Nexus 4 since launch I bought an LG G4 after thinking hard about the SG6, the M9 and the Z4.
I am really thrilled with the G4. The screen is amazing and the camera stunning. The author didn't mention one of the key points about it having a fully manual mode for shots - ISO, Shutter speed, EV, White Balance, 9 point auto focus or full semi-auto focus or complete manual focus. The opportunities for creating really high quality shots even in challenging conditions is great (combined with F1.8 Apeture!). The optical image stabiliser works superb and the slow mo video is good quality to.
Also charging speed after having a nexus 4 was an eye opener even with the standard charger (haven't tried the quick charge 2 charger yet).
I was anxious about the size after a Nexus 4 but it doesn't feel significantly bigger, in fact in many ways it feels smaller. I also like stock android and this doesn't dilute it too much - it's definitely plenty fast enough. Also compared to the Nexus 4 the GPS and compass are much more accurate - side by side in the car it was quicker to lock and remained much more stable. Even streaming radio on my journey in it didn't drop out once whereas the Nexus 4 had 3 blind spots on the same journey so the radios seem to be much better (LTE is non-existent around here so it wasn't for any lack of that on the Nexus).
The lack of water resistance/water proofing is a shame as I use a phone a lot in difficult conditions so I'm looking for a suitable case and the lack of a fingerprint sensor is a small omission. However these are nice to haves.
Another point that I hadn't seen mentioned, my G4 bought outright from an independent was a brown leather one but came in the standard pack with nice subtle gold back included as well and also came with some pretty reasonable headphones.
I might have some 'purchase justification bias' but I generally don't get too excited by phones as I deal with lots of different ones at work, however so far I am not displeased with this.
For a review from a real photographer see here: http://www.colbybrownphotography.com/gear-review-a-photographers-take-on-the-lg-g4/