I got one to replace my day 1 white iPhone 4 (yes I hold onto usable tech for a long time).
After installing and uninstalling a lot of apps and receiving a few OTA updates my Oneplus X battery life was totally miserable (~14 hrs normal usage). Reset to stock, did the latest OTA, let Google Play do it's thing and only install the apps I wanted = battery life is triple what it was at purchase date. I can now get a day and half's normal usage out of it.
Awesome phone for the price, the 1080p screen is truly beautiful, nothing else near 200 quid renders virtually Stock Android so nicely. Oxygen OS is so vanilla it's refreshing, I've never turned on the 'shelf feature' I like close to stock. Much nicer than any others manufacturer skins / custom launchers.
Buy a case with the phone though, it slips off of anything that isn't dead level, I really mean absolutely anything, think butter in a hot pan, thats how slippery it is. The free cover is ok but the kevlar one is an excellent design (especially at the corners where it grips the phone and also where you need the most protection).
The camera quality was terrible before the latest OTA now its a lot better + battery life is miles better. Roll on the marshmallow update for even better battery life and proper SD card storage merging.