The back button is grand for me as a lefty
In regards to lag in the keyboard are both your devices same carrier version as each other?
I'm wondering if this is specific to that carrier rom / firmware.
Battery life is average for me too. It gets me through the day but it doesn't excel at all. I leave mine on full auto brightness. If I don't it doesn't do the ramp up for outdoor visibility.
Fingerprint scanner is temperamental, but like yourself I set up my fitbit as a trusted device so don't have to deal with it most of the time. It likes my fingers more than my thumb. Noticeably prefers them.
Call quality is very good on mine. Making phone calls is not a problem with folks sounding clear and crisp.
Love adapt sound on Samsung devices for listening to music. The way it tests your hearing and the adjusts levels to suit your own personalised hearing is great.
Wireless charging is wonky for me. It seems to charge to about 92-98% and stop and then when it thinks it's charged it keeps waking screen up, chimes and then goes off and then wakes screen up and chimes again in a perpetual cycle. I didn't get my Samsung wireless charger yet as it comes separately, so will see if that fixes it.
The device isn't perfect by any means. Indeed when I'm relaxing at home I actually prefer to use either my nexus 6 or iPhone 6+ as my main browsing / media consuming devices. However my edge is great for when I'm mobile as its size is a perfect sweet spot for comfortable use as a phone.