The screen as well as being large is incredibly detailed and the colour reproduction is great. The rounded edges make it feel very comfortable in the hand and I find I use it by holding it in my left hand and type or gesture with my right hand. Despite the size it actually feels like it may be slightly lighter than the 4S. Some of the apple apps (e.g. Mail) now have a Landscape mode which presents more info and a better layout on the larger screen. iOS 8 has a slightly different feel to iOS 7 even though they don't look all that different. I was lucky I missed the debacle that was the 8.0.1 update which caused problems with phone calls and TouchID and went straight to 8.0.2.
I have been comparing the built in pedometer with my FitBit One and the phone always seems to come out with a lower number of steps but the amount can change wildly. When I been simply walking the iPhone shows about 2% lower but if I am walking around shopping and standing around then it can be about 10% lower. I think the Fitbit is perhaps overly sensitive as it shows me having walked steps when I am driving whereas the phone doesn't register any steps. The Fitbit is even worse if I'm not wearing it but leave it on the car seat next to the phone. So for the moment I'll use both but I suspect the phone will end up being more accurate. The steps are logged into the new health app dashboard which has potential to store lots of different health related info, including emergency info in case of an accident.
The camera seems very good and much more responsive than the old 4S. It has new slow-mo video modes and time lapse which I haven't tried out yet. The burst mode is sensational and takes approximately 10 photos per second while you hold the shutter button down. I tried it for 10 seconds, taking 100 photos with no slow down to write the photos to storage, not sure how long it would continue at this speed. This mode is a bit too easy to activate as I found keeping my finger on the shutter button just a little too long resulted in small bursts of 2 or 3 photos being taken but I suspect I just need to get used to this. The editing in the photos app is much better with more options and very responsive. The digital zoom seems to work reasonably although I suspect it is better to not use it but crop the image later but for a quick and easy shot it's not too bad.
At the moment the feature I am enjoying the most is the TouchID to unlock the phone. After setting up and calibrating with my fingerprints it is working accurately and quickly and makes using the phone for a quick check of something much faster. The only real problem I have noticed so far is that every day or two the wifi connectivity goes wonky and it won't connect to the network, requiring a restart. This has occurred on three different networks and I notice that the Apple Discussions Forum has a rather long post about this but it seems to be a iOS 8 problem and not a iPhone 6 problem. Hopefully it will be fixed in the next iOS update.
All up I'm very happy with the phone and glad I got the large one.