Within the scientific community, there seems to be very, very little doubt that a protein shake is extremely effective following a workout (cardio or resistance). There are very little products in the fitness and health industry that have such a solid backing behind it.
Essentially what happens is the following: When you workout, you put your body in a state of breakdown (ie. catabolism) due to all the physical stress it must endure. Straight after a workout, your body is still breaking itself down in order to recover. A protein shake provides the nutrients the body requires that would otherwise be obtained from the breakdown of muscle tissue. So - the protein goes into the body, the body absorbs the required nutrients and goes into an anabolic state (ie. a state of muscle building).
For cardiovascular work, a similar process reigns true. During the exercise, you place physical strain upon your body, breaking down muscle and fat tissue. Consuming a protein shake stops this breakdown process, kicks the metabolism into high-gear and allows the fat-burning process to effectively begin.
Also worth noting, during a resistance training workout, no muscle is built - muscle is broken down. The workout itself is a tool to be used in order to stimulate your body to build muscle mass. The same theory goes for cardio workout - a very effective form of cardio is high intensity cardio in order to stimulate your body to burn fat for the next 24 hours.
A diet should never be based solely on protein. There are fears (yet no conclusive proof) that a high protein diet can lead to kidney/renal disease in the long term. It can also contribute to osteoporosis, dehydration and excessive flatulence. When you are seeking to build muscle mass - carbohydrate, protein and fats should ALL be included in your diet in substantial amounts.
The amount of protein varies between individuals. It also highly depends upon the individuals goals. For example, a 60kg male will require far LESS protein than a 100kg male due to the 40kg difference in weight. In addition to this, someone seeking to build lean muscle mass will require MORE protein than someone seeking to lose body fat. At the end of the day, there is no specific amount of protein that is proven to be most effective in order to reach a specific goal due to everyone's genetic differences.