So when are sequels ever good?

The answer is never. Sequels are (almost) never good.

But why? Why are they (almost) always bad?

Because of expectation. Expectation leads to disappointment. Sequels never live up to our expectation because we expect to feel the same way we felt watching the previous one. Sequels to a bad movie have a chance to be good because we expect it to be bad. So if you saw a good sequel to a bad movie you would say to yourself, "Hey, that movie wasn't too bad..." But to a good movie you would feel disappointed and unhappy because it didn't live up to the caliber of the previous.

We feel bad about a sequel because we expect too much from the movie. Perhaps that is the case with this movie, Shanghai Knights the sequel to Shanghai Noon. Maybe my emotional baggage leftover from the previous movie biased me to this one thus making me feel bad about it. Bad about the quality of the script, the lack of chemistry between Jackie Chan and Owen Wilson. I needed a confirmation to my theory, so I've done an experiment. I've shown someone Shanghai Knights and afterwards Shanghai Noon. And they agreed with me that Shanghai Noon is a vastly superior movie.

In conclusion, skip this movie and rent Shanghai Noon again.