After having the phone 2 weeks, I now feel it’s adequate time to give my impressions.
By now everyone knows the specs, 2K screen, awesome camera, expandable memory... I’ll try focus on actually using the phone.
First impressions for me coming from a Z3 compact was this is a big phone, it’s definitely not a one handed phone unless you’re a basketball player but still very manageable for most tasks using just one hand.
The camera is amazing. The optical image stabilisation is a must have and the camera takes fantastic pictures in low light conditions. The combination of the large aperture (f1.8), optical image stabilisation and the laser focus has meant only a handful of pictures have come out slight out of focus, taking into account I’ve taken over 1000 pictures its truly impressive. With my Z3c I was having to retake pictures more often than I’d like to admit. Manual mode is fun to play with, I’d recommend a Joby tripod or similar if you’re planning on long exposures. Also take the pictures using one of the voice prompts as this will avoid any shake on the camera. The results you can achieve are stunning. Everyone who has played with the phone and viewed the pictures have been truly impressed. Displaying them on the stunning 2K screen also helps as the detail you can see is eye candy. Someone mentioned the double tap of the volume buttons not always working to fire off a "quick" picture without waking up the screen first, its worked everytime for me barring when you have headphones or bluetooth devices with volume controls connect as the volume buttons are obviously now set to control the volume.
Performance is snappy and lag free in normal day to day use. Everything opens quickly and transition between apps is super quick. The only time I ever had lag was when I was doing ALL of these at the same time, updating loads of apps, copying videos to the phones sd card, downloading my Spotify playlist and using the phone normally. Doubt any phone could handle all of that and still be super snappy. Even under load it handled all those tasks at once pretty well. As mentioned, during normal daily activities it is smooth as.
The knock knock feature for waking up the phone by tapping on the screen helps immensely due to the power button being on the back of the phone. While in your hand having the volume and power on the back is very convenient and better than the sides as they are where your index finger normal lies and you don’t end up pressing them while holding the edges, while the phone is flat on the table not so much. This is where knock knock comes in. TIP: You can also put the phone back to sleep by double tapping the top status bar or free space on any home screen. While the phone is unlock the volume options are easily accessible by pulling down the status bar but sadly these are not available on the lock screen so you’d still have to pick up the phone to change the volume which isn’t an issue but just an omission/oversight perhaps.
Battery life, the number one feature everyone wants but hardly any phone delivers... Over all I’ve been impressed, coming from a Z3c which was voted best battery life of 2014, the LG G4 is on par or better considering it has a 5.5 inch 2k screen vs the 4.6 inch 720p on the Z3c. On a normal day I’d be home in the evening with enough spare capacity I wouldn’t be worried it was going to die on the train home. I do use the phone a fair bit so that is impressive. On days I used the phone lightly I had > 50% by the end of the work day. Days I went out taking pictures nonstop I burnt through the battery in about 7 hours, which doesn’t sound great but I’m talking about taking 200+ pictures with hours of onscreen time in bright sunlight so the phone needed to bump up the brightness high. Taking that into consideration and other phones I have used would be dead in about 4-5 hours so it’s definitely got some stamina. You could carry a spare battery around if you really needed to as the rear cover is removable, would be handy on a holiday.
The plastic back does feel like plastic, surprise surprise. I like putting a cover on my phones so that’s just fine with me as I’d rather have a removable cover with access to the battery and memory card slot than a sealed back which you can’t see once the cover is on anyways. I haven’t seen the leather backed version of the phone so can’t comment. I think the leather back with a tampered glass screen protector (nillkin or the like) would give it a great look yet still feel its adequately protected.
Over all I think it’s a fantastic phone and have had great time testing it. Everyone who has used the phone has been really impressed with the screen clarity and camera quality which in my opinion are two mayor components phones need to get right these day. As mentioned before, battery life has pulled through and I haven’t had any days going home empty. Check out www.trylgg4.co.uk to view the photo archives of real people using the phone.
Forgot to add if you have any questions feel free to ask and I'll answer them as best I can.
. ....(and I am talking about eyesight and 'cronky' fingers). 
