Are knee push-ups any good?
You have to understand that the most important aspect of a good pushup is not just arm or chest strength, but the ability to perform full-body tension. You have to be able to make use of your body as a single unit, or create tension from your hands to your toes. The problem with knee pushups is that it eliminates the contribution of your whole lower body as it cuts the tension from the knees down and you don't actually learn the proper form through time.
The best way to start is to perform proper planks first to practice full-body tension: