(It's) pretty good; (it's) not as unpleasant as one may have originally thought. A: "I know it's your first time trying sushi. How do you like it?" B: "It's not half bad."

Somewhat satisfactory or good, especially compared to initially low expectations. I didn't know what to expect, but these corn dog tacos aren't half bad! A: "How was the concert that Kelly made you go to?" B: "Not half bad, actually. Those guys can actually play."