What is the reputation about manic depressive illness aka bipolar? are patients supposed to be wildly impulsive?

It all depends: Not likely. However, the truth is behavioural control of an individual.

Impulsivity: Without consistent treatment with mood stabilizers, then yes, a person with bipolar disorder can get sick & become impulsive. When manic, they can spend money without thinking, get involved in wild business schemes, stay up all night, talk rapidly, and change subjects suddenly. On the depressive end they can impulsively kill themselves. Treatment helps prevents these devastating outcomes.

It Depends: Bipolar disorder is characterized by erratic moods that can involve impulsivity during manic episodes, but can be reduced and possibly avoided with ongoing medical treatment by an experienced psychiatrist. For many patients who suffer from bipolar disorder, there can be periods of normalcy that generally are not associated with impulsivity (unless it is part of a different diagnosis).

Agree with Dr Pappas: Impulsivity is a common component of mania and may be represented by excessive spending, sexual indiscretion, or other risky behaviors, but not necessarily "wildly" impulsive.