Is diverticulitis serious?

Possibly. Diverticulitis is inflammation of the large bowel (also known as the colon). The inflammation results from simple infection to catastrophic perforation with leakage of contents into the abdomen. Most patients experience very minimal symptoms and are easily treated with antibiotics. However, some patients present with life-threatening symptoms, requiring hospitalization and emergency surgery.
It can be. Diverticulitis occurs when a small pocket on the wall of the colon ruptures. If the hole seals quickly, one may be treated as an out-patient with oral antibiotics. Often, the inflammation is more severe, requiring IV antibiotics in the hospital. Rarely the hole doesn't seal, requiring emergency surgery to remove the ruptured segment of colon.