How serious is pneumococcal pneumonia?

Serious enough. It depends on how severe. It can reach from a severe infection that causes severe lung injury, systemic infection with organ failure and a real risk of death, to a milder infection that one treats as an outpatient. It is always something however that requires medical attention and treatment (antibiotics, supportive care).
Very serious. The seriousness dependes upon how symptomatic patient is, how large are the pneumonic infiltrates, how is the oxygenation and her a-a gradient whether or not there is septicemia and the response to the medications. Usually responds to antibiotics.