A lot of people shared my excitement about Philip Glass’s and Christopher Hampton’s “Appomattox,” which, after seeing a second time on Friday night, I remain convinced is a superb opera. But a lot of other people didn’t. One of my respected colleagues, as he left the theater on opening night, Tweeted to his followers that the opera was so bad people should save their money and stay home. (We can only laugh together at the disparity of our reactions.)