Linux enthusiast here. Had a BQ E4.5 Ubuntu edition since April 2015 and I am pretty pleased with it. I love the UX.

It has come a long way already: the off the air (OTA) upgrades roughly every month make it better and better. It is more than usable. If not having Skype, WhatsApp etc.. is not a show stopper for you, then it can be a great phone for you. There are already many apps, but of course not comparable to Android. Can see an official app viewer here: https://uappexplorer.com/

Also the dev tools for it are great. The Ubuntu SDK, based on QtCreator is pretty slick. It is quite easy to write a native app. There are many guidelines and good docs for it: https://developer.ubuntu.com/en/