Pizza contains all the ingredients of a balanced meal and if cooked right with appropriate levels of ingredient can be quite healthy. The problem occurs when this does not happen. The average pizza is around 1,000 calories which is hard to incorporate into a dieting regime.
Pizza Express have a calorie count on their menu on their website showing that Pizza doesn't need to be calorific or fattening. For pizza and dieting try halving your usually portion and having this with a large salad.