I may comment on taxis another time, but I have just shy of nothing bad to say about the Beijing subway system. It is cheap. Trains run frequently. Most of the time, announcements are repeated in English and most of the directional signs are translated into English.
BUT, like nearly everything Chinese, you must share the experience with mobs of people. We have been on the subway when you couldn't possibly fall over no matter how suddenly the train stopped or started. On one memorable occasion, I could barely breathe because there was no room for my lungs to expand.
I believe the subway is just a microcosm of China as a whole. You constantly feel the pressure of all those people.