How do I respond to "how do you like your program" if I completely hate it?
"My experience hasn't been great" is a clean way to indicate the problem without criticizing anyone. You may want to couple this with something honestly positive -- such as "I really like my research topic, but overall, my experience in the program hasn't been great." 

If they follow up -- and they probably will -- you can say something like: "I've had a number of issues with the faculty. I probably shouldn't go into all the details though." In this way, you give a reasonable answer to their question without inviting further discussion. You can then redirect the conversation to something you're more positive about -- for example, your research or graduation plans.