Is Khalid a good general? If yes,when?

I actually think that he is a good general. His attack is good, he has good skills and he is not very expensive at all when we upgrade his skills. My opinion is that he and Bismarck are an excellent free pair of Art generals for any lineup, and I completed the game previous time using only this pair of Artillery generals. Of course, you can replace Bismarck with LiShimin with even greater effect.

It is true that his defense is weak and in comparison to Li Shimin his HPs are not impressive. But for an Artillery general it is not the most important point. If you can keep him at a distance his survivability will be OK. Much more serious is the fact that he looses 50% no-retaliation possibility at lvl 6. So all his attacks against towers will be retaliated (and taking into account that his defense is weak it is hardly inspiring). But we can remedy this by using one or two Cure Masters. So I strongly recommend to use either Caesar or Mocte (or even both) with Khalid in the team. Otherwise it will be better to use Artillery generals with a better survivability like, for example, above mentioned Li Shimin.