It depends on what type of meditation you are doing, what you visualize. For example, in the forgiveness meditation, we were trying to visualize in order to bring about compassionate understanding and forgiveness. So it really depends on what type of meditation you are doing, what you visualize.
What do you consider a good meditation session? Try not to use these types of concepts of good or bad regarding your formal practice.
In fact at the end of the 10-day retreat, I will often talk about how sometimes what people consider a good meditation is where they are able to bring up the power of their will-power and suppress all the thoughts away and get a calm feeling. But actually then they don't develop this gentle, allowing awareness that we are talking about. They don't know how to deal with their thoughts, so when these thoughts come up again they don't know what to do with them in their normal life.
So I would not consider it to be a "good meditation" just because we can get calm. And some people may consider a "bad" meditation to be when they have many thoughts arising and they are not able to get this calm blank mind. Yet if they are using the mental noting, trying to be more aware, seeing how thoughts arise, seeing how they pass, learning how they affect the body and mind, and developing more compassion to themselves and others, then that actually may be a good meditation. They start to develop this understanding of the law of cause and effect and they are able to deal with their thoughts in their normal life. So good and bad, if one is patient, if one learns how to develop this objective awareness, then perhaps I would say that is a good meditation.