The volume of this cup is divided into two equal parts; one a colorless, odorless liquid, the other a colorless, odorless gas. Thus the cup is neither full nor empty. Rather, each half of the cup is full, one with a gas, one with a liquid.
It's not about what you say but how you think of it. If you think 'Yes, there's still half left' then that's like saying half full, but if you think 'crap, there's only half left' that's like saying half empty.
I usually say "half-full", but if I'm looking at a glass, I'll may wonder whether someone drank some of it or just filled it half-way. Which is deep in and of itself. :P