pros - significantly reduces the risk of infection. By mandating that all be immunized it almost eliminates certain diseases. For instance Polio was erradicated in the United States.
cons - there is no such thing as a risk free immunization. Now the case is that the risk of having a side effect from the immunization is much lower than the chance of contracting or dying from the infection you are being protected against. There is always a risk of side effects from the components of the vaccine, and if it is a live attenuated vaccine there is also a chance that the virus will revert back to a infectious form once in the body.