Jolla is the last company with an 'alternative' mobile operating system
I'm using Sailfish OS on an Xperia X since when it became available. My main reason to migrate from Android was "battery life" which was confirmed (battery now lasts ~5 days instead of the usual 2-3), but the OS has a lot of small and bigger bugs (e.g. carrier selection, 2G/3G/4G priority, stuck data comm for Android apps) and missing features (notoriously: tethering/wlan access point, bluetooth) that do not currently make it a valid alternative for a normal user if compared to the other operating systems. Version 2.1.4 beta is available (https://blog.jolla.com/sailfish-os-2-1-4-now-available-early...) which might fix many bugs and deliver some key  features, but the date for the final release has not been published (and I'm too scared to brick my phone by installing the beta).