Is heartburn a serious condition?

It can be. Heartburn is a common condition, affecting millions of americans. Most of the time this is mild and not serious, but heartburn can dramatically affect quality of life such as by reducing effective sleep. It can lead to complications such as strictures in the esophagus (swallowing tube) and even increase the risk of esophageal cancer.
Yes. Typically, intermittent heartburn results in no lasting problems. However, there is a risk that the chronic inflammation of the lower esophagus caused by longstanding reflux can lead to precancerous changes (dysplasia) and even progress further if undiagnosed/treated.