How effective is cardioversion for helping af?

Effective. If the patient is an appropriate candidate: symptomatic, af unresolvable by medication only, other medical factors the cardiologist will consider, then yes, cardioversion is effective for regaining normal sinus rhythm. Like all therapies, it is not 100% effective and must be evaluated for each individual patient.
A good start. Cardioversion is a great first step in managing AF. The procedure is generally very low risk and is highly effective at restoring normal rhythm. The hard part is maintaining normal rhythm. The heart tends to want to go back into AF without rhythm controlling medications or elimination of the trigger for AF (such as excess alcohol, uncontrolled blood pressure, sleep apnea, etc.).
Cardioversion. The shorter the duration of atrial fibrillation the more effective cardioversion is at converting the rhythm. When underlying cardiac pathology like mitral insufficiency or stenosis is present, keeping the patient out of atrial fibrillation or converting the rhythm is quite problematic.