I've switched over to Ubuntu Touch recently and have Ubuntu Touch 16.04 Stable on Nexus 5.
So far I love using it as my daily driver, but whenever I have to use a calculator, I find myself pulling out my old(?) iPhone. The only reason is because the Calculator on UT has no [-/+] button to flip the final value.
I assumed it gets utilized quite a lot in general, and was surprised to see its omission on the Calculator. I often calculate series of usages in % and the fact that I have to run back for my old phone or an actual calculator during workflow is a bit disappointing.
Hope the team will look into this.