The glass is full: This beer is excellent! I'll drink this beer whenever I have the chance. Highly recommended.
The glass is half full: This beer is good. I'll have another please.
The glass is half empty: (which is indeed the same as half full literally - but not rhetorically) This beer is just okay. Don't think I'll have another.
The glass is empty: I would only drink this beer if I had nothing else available, or if I really didn't care, or if it was dirt cheap. And maybe not even then. This is swill.